Belt Tension Theory

Driving or slack side tension (T2)

To determine the minimum tension which must be introduced into the belt as it leaves the driving pulley, to ensure that the

effective tension or power can be transmitted into the belt passing around the drive pulley, without slip.

Prior to going into this analysis, it is necessary to understand the effect that friction and wrap angle have on the driving

characteristics of a pulley.

These are discussed below :-

(a) Friction Factors

The fundamental equation for conveyor drive calculations is: T1 / T2 = eθ

Figure 1: Typical Drive Pulley

Where:

T1 and T2 are the tight and slack side tensions respectively.e = 2,718θ = angle of wrap in radians

= coefficient of friction between belt and drive pulley

The tensions and angle of wrap are fairly tangible and are determined by the configuration of the machine and the loads on it. , however, is not well understood, difficult to measure and subject to wide variation with different materials and

environmental conditions.

The following factors will determine the selected coefficient of friction :-

(i) Belting Material

The shore hardness, type of rubber and the age of the belt will have an effect on the coefficient of friction.

(ii) Drive Pulley Construction

This can be a bare steel pulley or it can be lagged to increase the coefficient of friction. The lagging can have different shore hardness, which changes the coefficient of friction. It can be grooved, which improves the coefficient of friction in certain wet conditions, but again it must be remembered that the coefficient will reduce in time because of the ageing of

rubber and the blocking of the grooves in the rubber lagging, or due to wear on the lagging.

Click here to go to the pulley section where more information can be found on: Pulley types and Pulley lagging .

(iii) Working Conditions

The conditions under which the installation will operate, i.e. will conditions be dry or wet, and if wet, to what extent? What size and what type of material? Fine powdery material can reduce the coefficient of friction by acting as a lubricant

between the pulley and the belting. If the material to be handled is oily, it will reduce the coefficient of friction drastically

especially under wet conditions.

(iv) Design Conditions

The following design conditions will effect the coefficient of friction :-

Angle of wrap of the belt around the drive pulley. The maximum tension to be transmitted via a drive pulley

The maximum face pressure between the belting and drive pulley. This will not only effect the

coefficient of friction but will also limit the selection of the type of lagging for a pulley.

Figure 2: Force Distribution on Pulley Face

For example, in figure 2 above :-

Face pressure PF can be calculated as follows

Fn = 2T Sin dθ/2

PF = Fn/r d = T/r with dθ = 0

where:

PF = face pressure kPar = pulley radius (mm)Fn = force on pulley, kN/mTave = belt tension, kN/m

θ = angle of wrap, degrees.

Extreme pace pressure would result in peeling and rolling away of the pulley cover / lagging, especially at the drive pulleys

where there would be belt creep.

High tension steel cable belts are usually the only place where face pressures become a factor in lagging, pulley and drive

selection.

Soft rubber lagging is not usually suitable for drives of more than 100 kW per pulley and slide-lagging is usually limited to

drives of 400 kW per pulley, but the lagging thickness will have an obvious effect on this. 

Diameter of pulleys must be selected to minimise the face pressure for rubber to 1 000 kPa.

Minimum Pulley Diameters: Fabric Belting or Steel Cord Belting 

b) Drive Factors

Most references publish tables of drive factors for various values of and this tends to obscure the fact that is faily arbitrarily

chosen.

The drive factor, 'Cw' is defined as

Cw = 1 / eθ - 1

and

T2 = Cw Belt Power (kw) / Belt Speed (m/s)

T1 T2 = Te

and

T2 = CwTe

The use of drive factors is to be discouraged. Designers are urged to select carefully for each application.

Values of published by various authorities are shown in Table 1 below:

Table 1: Typical Friction Factors for Drive Pulleys

CONDITION CEMA STEVENSADAMSON

BRIDGESTONE LINATEX REMA TIP TOP

Bare Pulley 0.25 0.35 0.1 - 0.3 -- --

Lagged 0.35 0.35 -- -- --

Dry Lagged 0.35 0.35 0.35 0.6 0.4 - 0.45

Wet Lagged 0.35 0.35 0.2 - 0.3 0.8 0.35

Wet and Dirty Lagged 0.35 0.35 0.2 0.4 0.25 - 0.3

These values are the limiting conditions (when the belt is on the point of slipping). The actual coefficients of friction

developed between surfaces are, in practically all cases where slipping does not occur, in excess of those shown above.

Therefore, the convention of using these values, whilst convenient, does not reflect what exactly occurs at the drive.

With a working drive, the higher tensioned part of the belt is stretched more than the lower tensioned section, thus the belt

entering the positive drive will be travelling faster when it leaves it. (In a regenerative drive the opposite is true).

The elastic recovery of the belt occurs over only a part of the total angle of contact and it is at this point, (where creep

takes place), that all the driving is done, while making full use of the coefficient of friction.

It is only over this 'active part' of the wrap angle (See fig. 4 below) that the classical tension formula can be applied, because this is where the limiting condition occurs. This is an approximate solution as it varies with both creep and angle of

wrap.

A typical graph of '' against creep rate is shown below in Figure 3.

Figure 3: Graph of '' against Creep Rate

Figure 4 shows the drive conditions where the 'passive' arc is between 'A' and 'B' and all the drive is transmitted between

'B' and 'C' which is called the 'active arc', with '' being dependant on creep rate.

Figure 4: Diagram indicating 'active' and 'passive' arc on a drive pulley.

By maintaining a constant T2 and increasing T1 the active arc will increase, until it uses the whole of the wrap angle. A further increase in T1 will cause slipping and a reduction in the power transmitted. To realise the importance of the

variations in '' consider the following:

Comparing =0,2 and 0,4 for θ=3,66 radians then;

T1/T2 = 0,27 and T1 / T2 = 4,32 respectively

i.e. for the same T1, T2 will have to be more than doubled to drive without slipping at = 0,2.

c) Rubber Specification

Except for Linatex, which is a special material, most references mention no particular rubber specification. Also, very little

attempt is made to allow for conditions such as dust which must affect drive performance.

According to figures quoted by Bridgestone we should use = 0,2 for wet and dirty application instead of the conventional 0,35. This value applies only in the limiting case and the Bridgestone factors are very conservative. This has been verified by Stevens Adamson, who claim that the actual coefficient is likely to be in the range of 0,5 to 0,6 but 0,35 is used for safety and to allow for high starting torques. No distinction is made for chevron lagging as the grooves are for drainage

only.

It is interesting that very little attention is paid to the actual rubber material with which the pulleys are lagged. The coefficient will vary with the hardness and type of rubber on the pulley and on the belt as well as with external, or

environmental conditions.

The standard rubber lagging used has a hardness of 55 shore and this is supposed to be a good compromise between grip and long life. However some companies insist on a much harder material in the range of 65 to 70 for long life. Rema Tip

Top use 65 to 70 for heavy duty drives but go down to 50 for lighter duties.

Softer, more cohesive rubbers are used, but there is a greater tendency for the rubber to shear internally and eventually

pull off. Coefficients claimed for soft rubbers are shown in Table 2 below.

Table 2: Typical Values of for Soft Rubbers

  Shore Hardness Dry Wet Wet and Dirty

Rema Tip Top 50 - 55 0.4 - 0.35 0.35 0.25 - 0.3

Linatex 60 0.6 0.8 0.4

The thicker Linatex lagging actually has a fabric layer in the rubber to prevent internal tearing from taking place. The

conclusions which can be drawn from the above are:-

i. Published tables must be used with great care. Careful consideration must be given to the materials involved and the conditions under which the machinery will operate. It is not good enough merely to

look up the 'drive factor' in your favourite catalogue i.e and then press on regardless. ii. Figures published by CEMA, Bridgestone, etc., are conservative and there is scope for intelligent use of

higher factors to reduce belt tensions.

iii. Materials such as Linatex are available which have a much higher value than the usually accepted

values.

iv. If too low a value is chosen for the conditions or the conditions are misinterpreted, drive/belt slippage

may result and serious problems will occur.

Finally, table 3 lists the coefficient of friction which could be used for future designs. This table can be expanded to include

Ceramics.

Table 3: Coefficient of friction - Drive Pulleys

Plant Description Conveyor Construction

Type of Take - Up's

Automatic Manual

Lagged Un-Lagged Lagged Un-Lagged

Wet Plant Covered ConveyorUncovered Conveyor

0.250.20

0.100.10

0.200.20

0.100.10

Semi-Wet Plant Covered ConveyorUncovered Conveyor

0.300.25

0.200.15

0.250.22

0.180.13

Dry Plant Covered ConveyorUncovered Conveyor

0.350.30

0.220.18

0.250.25

0.200.15

With an understanding of the factors effecting the slack side tension T2 it is now possible to continue.

Having calculated the driving or slack side tension from the above analysis.

The resulting tight side (maximum tension) (T1) in the belt at the drive pulley would be :-

T1 = TE + T2    (N).

Sag Tension (Ts)

In many instances the slack side tension as calculated above will be sufficient to ensure that adequate tension is retained at all points around the conveyor. However, in some cases, it is found that if the calculated T2 is used the belt tension at

the tail end of the conveyor is not sufficient to support the loaded belt without undue sag between the idlers.

Under normal circumstances the sag between the idlers is restricted to 2 % of the idler spacing. The tension required (Ts) is

dependent upon the mass of the loaded belt and the idler spacing and is given by the formula :

Ts = 3 x %sag / 100 x IS(carry) x (Wb + Wm) x 9,81 with units of  (N)

Therefore it is necessary to carry out a check calculation, taking the tension required to limit sag to 2 % and comparing it to the tension to prevent belt slippage at the drive pulley. If the sag tension is found to be too low to support the load then the T2 tension and thus the tension distribution throughout the conveyor must be increased by the difference between the two

tensions.

Maximum Belt Tension (T1)

To calculate the maximum belt tension, which does not in all cases takes place at the head pulley, the following formulae

must be used :

TI = T2 + TE  (N) or,

should Ts be greater than T2 then Ts must be used in place of T2

T1 = (T2 - Ts) + T2 + TE  (N)

Belt Tensions/Strength: Fabric Belting or Steel Cord Belting

Unit Tension Conversion

To conclude, the total belt tension as calculated above is usually expressed as a unit tension for reference against various

information tables. This is done by dividing the total calculated tension by the belt width.

Therefore Unit Tension = Belt tension (N) / Belt width (mm) giving units of N/mm width.

Belt covers and carcassCharacteristic Possible problem Solution

Top cover wears quickly High acceleration or side loading of material in loading area

Check conveyor speed. Direct stream of falling material in conveying direction with a

deflector plate

Dirty or sticking idlers Use return idlers with rubber cushion discs, lubricate or replace

sticking idlers

High sag between idlers causes relative movement of material

Increase pretension, reduce idler spacing

Cover to thin Specify thicker top cover

Rubber grade not adequate Choose grade of cover rubber according to material

Severe local wear, longitudinal grooves or damages of top cover

High pressure of seal skirts or chutes on belt surface

Adjust seal skirts perpendicular to belt, modify chute

Seal skirts consist of hard or wear resistant material

Use soft rubber with normal or low wear resistance (do not use

old belts)

Steel sides of chute to close to belt

Adjust distance to belt, increase distance in direction of belt travel

Excessive space between belt and seal skirts

Adjust seals to minimum clearance

Belt sags under impact, allowing material to be trapped under

skirts

Decrease idler spacing, install impact idlers or impact bars

High material volume causes Increase speed, reduce capacity,

friction between chute and belt surface when dragged out

modify the chute (the chute should become wider in direction

of belt travel

Material comes behind chute Improve loading, install baffles

Material jam in chute Avoid overloading, reduce material size

Puncture by tramp iron at chute Use metal detector or magnetic separator, use rip detection

system if necessary

Belt cleaners (scrapers) not adequate or adjusted to hard

Adjust or replace scrapers

Severe damages (longitudinal cuts, chamfers, gouges) of top

cover in belt center

Impact of lumps falling on the belt at loading area or high

material velocity during loading

Reduce drop height or velocity, use impact idlers, install impact

plate above loading point, choose impact resistant belt

Top rubber cover hardens Material or ambient temperature too hot

Install belt with sufficient heat resistance

Aggressive material drains softener from belt

Choose grade of rubber cover according to material

Small, frequent cracks in top cover rubber along the whole belt

Extreme influence of ozone and/or UV-light

Cover conveyor or protect belt from sun or UV-rays, install belt with superior ozone resistance

Top cover softens Material contains oil or grease Use oil and grease resistant cover

Lubricant has come in contact with belt surface

Check grease seals, clean belt

Severe wear of bottom cover rubber

Slippage on drive pulley Increase tension, install pulley lagging (grooved if wet), increase

angle of wrap at drive pulley

Material has built at surface of drive pulley

Capsulate loading area, install pulley scraper, install decking

between top and return strands

Excessive tilt of troughing idlers Allow maximum 2° tilt

Damages of bottom cover rubber Material trapped between belt and tail pulley

Install V-scrapers in return strand ahead of tail pulley, install

decking

Small frequent cracks in parts of bottom cover rubber along the

whole belt

Belt or bottom cover to thick / troughing angle to high

Install belt with appropriate design, decrease troughing angle

Belt edges wavy "butterfly" Edges overstressed due to short troughing transition lengths

Increase distance from pulley to troughing idlers, elevate pulley above level of centre troughing

idler, install belt with lower elastic modulus

Edges overstressed in convex curves

Reduce troughing angle in convex curve, reduce belt tension in

curve (beware of compression of belt centre), increase curve radius, install belt with lower

elastic modulus

Belt edges wavy "butterfly" in concave curves

Curve design, belt type and belt tension not well adapted

Reduce troughing angle, increase belt tension, install belt with lower

elastic modulus

Separation of top cover from carcass

Cover cuts or very small cover punctures allow fine material to

work under cover

Improve impact situation, choose belt with thicker top cover

Pulley diameters too low Install bigger pulleys, install more flexible belt

Separation of bottom cover from carcass

Extreme dynamic processes causes rubber fatigue

Avoid frequent flexing or high pressure, reduce belt speed,

increase pulley diameter, install belt with dynamically stronger

rubber grade

Pulley diameters too low Install bigger pulleys, install more flexible belt

Separation of textile plies Belt damage has allowed access of humidity and/or material

Repair damage, improve impact situation, avoid cover damages

Pulley diameters too low Increase pulley diameter, install more flexible belt

Separation of textile plies in idler junction

Troughing angle too high Increase troughing angle, use belt with higher troughability

Belt affected by large gap between idlers

Decrease gap between troughing idlers, install belt with higher transverse stiffness/thicker

carcass

Low transition lengths Increase transition lengths, elevate pulley

Low radius of convex curve Increase curve radius, reduce troughing angle in curve, reduce

idler spacing in curve

Separation of rubber cover or textile plies at belt edge

Belt edge has come in steady contact with conveyor frame

Improve belt steering

Chatter marks on belt surface Belt cleaner not well adjusted or damage

Adjust or replace scraper

Chatter marks on belt edge Belt edge has come in steady contact with conveyor frame

Improve belt steering

Crosswise cracks of carcass or textile plies, while covers are

intact

Low pulley diameters causes extreme flexing of belt carcass

Increase pulley diameter, install more flexible belt

Material trapped between belt and tail pulley

Install V-scrapers in return strand ahead of tail pulley, install

decking or guards

Lengthwise cracks of carcass or textile plies at belt edge, while

covers are intact

Belt has folded up due to contacts with conveyor frame

and ran around pulley

Improve belt steering

Belt has folded up after forming an "Omega-wave" and ran

around pulley

Check transition lengths, check convex curves, install more

crossridgid belt

Longitudinal cut trough whole belt

Jam of material or tramp iron Improve loading situation, install magnetic separator, install rip-

detection system

Belt has come in contact with conveyor frame due to

misalignment

Improve belt steering

Rupture of pulley rim Replace pulley

Defective idlers has fallen from its support and belt dragged

over steel edge

Replace and fix idler on the support

Belt edge damaged ot torn off Belt has come in contact with conveyor frame due to

misalignment

Improve belt steering

Belt spliceCharacteristic Possible problem Solution

Splice fails (rupture of reinforcement)

Belt tension too high Modify conveyor to reduce the belt tension (e.g. increase speed,

reduce running resistance, increase friction between drive

pulley and belt)

Splice fails (separation between splice steps)

Old or wrong splicing material Resplice belt with correct splicing material, do not exceed maximum storage time

Rubber solution not evaporated sufficiently

Resplice belt, let rubber solution evaporate well

Ambient conditions during splicing too humid

Resplice belt, provide rain protection, heat belt ends (max.

80°C) before final assembly

Splice opens in belt centre Compression of belt centre in transition area

Increase transition length, elevate the pulley 1/3 of the

trough depth, increase pretension

Splice opens or break at belt edge Tension at the belt edge to high Increase transition length, elevate the pulley 1/3 of the

trough depth, reduce pretension

Splice opens or breaks at the bottom side

Compression on the bottom side Increase pulley diameter

Fasteners pulling out Wrong type of fastener or fasteners not tight

Refasten belt with right fastener, retighten new fasteners after a

few hours run

Tension to high Change type of fastener, decrease belt tension

Transverse breaks in the belt right behind fasteners

Fastener plates too long for pulley diameter

Use smaller, shorter fasteners

Fasteners too big Increase pulley diameter

Cover strip separates from splice Pressure during splice too low Replace cover strip, increase pressure of vulcanizing press

Cover strip to thin Replace cover strip by thicker type

Joint sections of rubber covers cut too steep

Replace cover strip, cut cover less steep

Joint sections not ground Replace cover strip, grind/roughen joint section

Uneven transition allowed belt cleaner to rip off cover strip

Belt behaviour on the conveyorCharacteristic Possible problem Solution

Conveyor running to one side at a given point on structure

One or more idlers not rectangular to the longitudinal

axis of the belt

Adjust idlers

Sticking idlers Clean idlers and belt, install belt cleaning

Conveyor frame or structure not level or crooked

Level the structure

Guiding not sufficient Tilt idlers and/or install belt guiding

Particular section of the belt runs to one side at all points

Belt splicing not straight Resplice the belt.

Bowed belt A new belt should straighten out after operated under full load

tension, use self-aligning idlers on the return strand ahead of the

tail pulley if necessary

Loading being placed on the belt off-centre

Adjust chute and loading conditions to place load in the

centre of the belt

Guiding not sufficient Tilt idlers and/or install belt guiding

Conveoyor belt runs to one side at all points

One or more idlers not rectangular to the longitudinal

axis of the belt

Adjust idlers

Sticking idlers Clean idlers and belt, install belt cleaning

Sticking or blocking guide idler set

Maintain the guide idler set movable or replace it

Rubber cushion discs worn out or loose

Exchange rubber cushion discs

Conveyor frame or structure not level or crooked

Level the structure

Side loading Load in direction of belt travel, in center of conveyor

Conveyor belt runs to one side for a longer distance

Missing steering devices on return strand

Install steering devices, e.g. throughed roof idlers in angle of

10-15° on the return stand

Material trapped between belt and tail pulley

Install V-scrapers; install decking or guards

Material builds up at the pulley Remove accumulation, install scrapers, install decking or

guards, improve loading

Not parallel installed scraper on return strand

Check the position of the scraper; change worn scraper blades

Rubber sealing of the chute is not correct installed or not

symmetric

Install sealings perpendicular to the belt surface, change worn

sealings

Loading being placed on the belt off-centre

Adjust chute and loading conditions to place load in centre

of the belt

Belt running to one side at or close to the pulley

Pulley is not rectangular and level to the centre line of

conveyor

Level the pulley, install guide idlers or/and limit switches

Material trapped between belt and tail pulley

Install V-scrapers; install decking or guards

Material builds up at the pulley Remove accumulation; install scrapers, decking and/or guards

and improve loading

Pretension too low Recalculate and increase counterweight or screw take-up

Belt misalignment, following no pattern of performance

Belt too stiff to train Reduce the troughing angle, tilt troughing idlers forward but not

more then 2°

Belt to flexible in cross direction Use belt with higher rating or

thicker carcass

One or more idlers not rectangular to the longitudinal

axis of the belt

Correction of idlers

Sticking idlers Lubricate or exchange idlers, improve maintenance

General slippage Pretension too low Recalculate and increase counterweight or screw take-up

Unsuitable pulley lagging Check the condition of the pulley lagging. In wet conditions, use

grooved lagging

Worn pulley lagging Change the pulley lagging. Use grooved lagging in wet conditions

No pulley lagging and dirty pulley

Install a pulley lagging and correct cleaning devices.

Increase the angle of wrap, install pulley scraper

Overloading Decrease the load

Material build-up Remove accumulation, install cleaning devices, scrapers and

inverted "V" decking

Sticking idlers Lubricate or exchange idlers, improve maintenance

Belt edges lift off idlers Chemical change of top cover, caused by material

Choose grade of cover rubber according to material

Bottom cover too thin or carcass thickness too low

Install belt with thicker carcass or bottom cover

Belt tension too high Modify conveyor to achieve a reduction of belt tension

Belt center lifts off centre troughing idler

Belt to crossridgid Install belt with better troughabillity, decrease troughing

angle

Belt center lifts off centre troughing idler in concave curves

Curve design, belt type and belt tension not well adapted

Reduce troughing angle in convex curve, increase curve radius, install belt with lower

elastic modulus

Belt Conveyor Technology

Belt Speed and Tension Control on Long Conveyors

10.11.2011 | Author / Editor: N.K. Romani / Marcel Dröttboom

PDF | Recommend | Save | Print

Distributed drive systems have enabled the installation of longer, economical conveyors (Pictures: Baldor)

The ability to control the drive acceleration torque providing a smooth soft start while

maintaining belt tensions within specified safe limits is critical for belt conveyors

performance. Soft controlled start drives on a long conveyor with multiple drives protect

the belt and other conveyor components and help reduce investments costs.

In the good old days, a single horse driven cart was extended to two, four or six horse stage coach to

cope with the increasing demand of passenger load. To ensure comfort to passengers from a sudden

starting ‘jerk’, the horses were trained to move the coach very gently and gracefully. And when the

horses galloped to full speed, they were whipped and controlled to ensure proper load share, thus

avoiding an overturn or failure of components. Similarly, when belt conveyors grow longer, from 1 to

2 kilometres to 8 to 15 kilometres and haul 3000 to 4000 tonnes per hour over significant inclines and

declines, the complex profiles place unusually stringent demands on the starting/stopping procedures,

drive configuration on primary and secondary pulleys at the head end, tripper stations in the mid-

sections and the booster station at the tail end.

Belt Velocity and Acceleration Control

A conveyor belt is a non linear visco-elastic medium in which strain can propagate at some finite wave

speed. Long conveyor belt is a gigantic rubber band full of elasticity and load inertia dynamics which can

result in destructive consequences if not properly understood. Perhaps the most difficult conveyor

characteristics to overcome are the velocity and tension waves present in the belt. In a conveyor with

head drives only, these waves tend to propagate to the slack side of the belt where they are attenuated

by friction and the take-up mechanism. When a fully loaded conveyor belt is started abruptly or with no

acceleration control, high acceleration forces induce tension waves that can adversely affect the belt

fabric, belt splices, drive pulleys, idler pulleys, shafts, bearings, speed reducers, motors and couplings.

This can cause performance problems in vertical curves, excessive belt take-up movement, loss of drive

pulley friction, spillage of materials, and festooning of the belt fabric. Therefore the drive system must

produce a minimum torque powerful enough to start the conveyor, and controlled such that the

acceleration forces are within safe limits.

GALLERY

Select a picture to open the gallery (9 images)

The ‘break away’ is the most crucial zone of conveyor going into motion from zero speed. There should

be no ‘jerk’. Generally, drive systems offer a linear velocity ramp (to minimize static acceleration force),

or do not attempt to attain any particular shape of the velocity ramp. To understand better the

velocity/acceleration ramp relationship, let us assume a conveyor starts with velocity increasing linearly

at 0.1 meter per second. The acceleration will remain constant at 0.1 m/s2. After 40 seconds the belt

attains its full speed of 4 m/s. At this point the acceleration will drop to zero. This is not a good method

of starting a conveyor as the jerk at the very first instant will be a huge spike, then remains constant,

which could be detrimental to conveyor performance. If the belt splice section happens to be at the

drive pulley at the instance when a fully loaded belt is started abruptly, the starting jolt can rip the belt.

On the other hand, if velocity is controlled such that it makes an ‘S’ shape curve (Fig. 1), the

acceleration curve will be sinusoidal. The jerk curve, in this case, is continuous except at the beginning

and end of the acceleration ramp.

V(t) = V/2 · (1 – cos(π/t ·T))

where:

V = Design running speed of the belt [m/s]

t = time [s]

T = time to accelerate the stationery belt to speed V [s]

Starting the conveyor this way will have almost no ‘jolt effect’, thereby no chances to propagate velocity

and/or tension waves. It should be observed closely that when velocity is 50 per cent, acceleration is

100 per cent whereas the jerk is zero. This is the zone where motor will deliver its maximum power or

torque, thereafter, as velocity continue to increase to reach 100 per cent speed, acceleration goes down

sinusoidal and jerk goes negative. It has been shown [1] that having an ‘S’ shaped velocity curve (or a

sinusoidal acceleration curve); the belt safety factor can be reduced by approximately 15 per cent. For

long conveyors, this reduction will typically yield cost savings that exceed any price premium for the

right drive system. The belt cost usually constitutes at least 25 to 35 per cent of the total investment on

a major conveyor project. If you are putting a 10 km long conveyor, as a thumb rule of 1 million USD per

kilometres, you are investing roughly USD 10 million. The belt itself may cost around USD 3.5 million. By

using an ‘S’ shaped velocity curve, you can switch over to a belt of next lower ST rating, saving 10 to 15

per cent on the cost. Besides this, having to have the peak belt stress, peak motor starting torque and

peak tensions reduced by 15 per cent, a significant reduction in belt weight leads to selection of more

economical idlers, less stringent specifications for the pulleys, as well as smaller motors, drive systems,

couplings, holdbacks etc., which finally yields tremendous amount of cost savings [2].

The need for longer velocity ramps has led to the “30 seconds per kilometre (of conveyor length)”

thumb rule. For belts 1 to 3 kilometres, the ‘S’ velocity ramp time is generally kept anywhere between

40 to 100 seconds, but for conveyors exceeding 3 kilometres, the results of the dynamic analysis can be

used for determining an acceptable velocity ramp time.

Belt Speed and Tension Control on Long Conveyors

10.11.2011 | Author / Editor: N.K. Romani / Marcel Dröttboom

PDF | Recommend | Save | Print

Therefore, an important design aspect in long conveyors is Dynamic Analysis. This is a mathematical

procedure that determines the belt‘s elastic behaviour to time transient force perturbations, such as

starting from motors, brakes, take-up pulley ramming into its stop, belt jams, etc. Long multiple drive

conveyors need to be carefully analyzed to ascertain belt behaviour during start, stop, and emergency

stop, under empty / partial load / fully loaded conditions. Results of such analysis will often help

determining the optimum drive configuration. This also helps in determining the belt speed ramp to

optimize the belt rating.

Stopping Control and Power Loss Condition

So far we have talked about method of controlled starting of long conveyors with multiple drives. But,

stopping of large high modulus belt is potentially more damaging, is less controllable, and is more

difficult to assess than the action of starting. More attention should be given to the forces generated

during stopping. While stopping, we could use a reverse S shaped curve which will again minimize the

stress waves induced in the belt. Often times, however, operational requirements demand a much

shorter stopping time than the starting. As a thumb rule, the stopping is 1/5 th the starting time. To this

end, the stopping algorithm is often simply a rapid controlled decrease in drive torques such that

tension waves are kept within reasonable bounds. After the torques are reduced to zero, the belt is

allowed to coast to a stop or some times brakes are applied to soak up the residual energy.

In a power loss situation the controls usually lose all ability to influence how the enormous energy

stored in the belt will be released. Catastrophic belt failures are possible under these conditions. The

principle problem is sudden release of torque by the drives. This will produce large tension and

compression waves which can over-stress conveyor components into failure. Often, inertia in the form

of flywheels is added to the drives to moderate the rapid release in tension. CST drive is equipped with

appropriate flywheel on its input shaft and the hydraulic control is designed such that during a power

outage, the clutch is fully locked. The flywheel inertia via the gear box is transmitted to the conveyor,

allowing it to drift for a while and coast down to a full stop without inducing tension waves.

GALLERY

Select a picture to open the gallery (9 images) Conclusions

On long conveyors, aperiodic tension waves caused by very erratic load switching and oscillating load

share among multiple drives must be contained to prevent the failure of belt splices, drive system

reducers, and other mechanical elements. A very low reflected inertia of the drive is the key to

successful control and management of the aperiodic tension waves. It is necessary that the drive torque

control element has a very low inertia, and is located at the output of the drive. The CST hydro-viscous

clutch, which controls the torque is located at the output of the of the drive system. The dynamic

responsiveness of this torque-limiting feature eliminates the possibility of a major load surge on the belt

conveyor passing through the torque limiting section into the reducer and motor.

Distributed drive systems have enabled installation of longer, economical conveyors. In order

to maximize its advantages, the sizing, selection and location of such drives and their control system

need careful design. The drives chosen have also to be amenable to the desired control to ensure that

the designed objectives are practically achieved. Several such conveyors are in operation with Baldor

CST drives and have proved to be very economic and reliable with significant benefits in investment and

operating costs.

References

[1] Harrison, A.: Modern concepts in belt conveyor engineering: Propagation of dynamic stress in high

modulus belts. Int. Materials Handling Conf. Johannesburg, South Africa 1985.

[2] Nordell, L.K., Ciozada, Z.P.: Transient belt stresses during starting and stopping; Elastic response

simulated by finite element methods. bulk solids handling Vol. 4 (1984) No. 1, pp. 93-98.

Noshir K. Romani, Baldor Electric Company, USA