985601 Vibration

7/28/2019 985601 Vibration

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Cummins Diesel

7/28/2019 985601 Vibration


7/28/2019 985601 Vibration


Table of Contents

Page

1istory of Vibration

2ngine and Vehicle Changes .

Linear VibrationEngine.ccessory. ...

Outside Origins

8

14

16

Resonance 18

21

25

26

Torsional Stress

Origins and Control. ,

Indications of Excessive Torsion

Damper Failure ,

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1

That's something engine people -

shop men as well as engine research

people- have heard a lot about in

the past 10 or 15 years.

Is it something that just developed

in the past 10 or 15 years, then?

No! Vibration is just another word

for movement -and it's a reasonably

sure bet that there was a little "move-

ment" in a group of vehicles likethese!

Even just thirty-odd years ago, it

looks as if this repower job may have

suffered a little from "movement".

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We've made a great many steps

forward, all of us; so why is vibration

still a problem?

There are a number of reasons.

First and foremost is that engines are

a lot different, too.

As the horsepower requirements goup, engine speeds and pressures have

to be increased.

And while hosepower has in-

creased, engine users have de-

manded smaller, lighter weight

engines.

Even engine shapes have had to

change some -the very popu lar ,

high powered V engine arrangement

fits all the other requirements better ,

but is sometimes prone to vibration

because of higher speeds.

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3

Trucks and off-highway vehicles -

have changed a number of ways.

They are made of lighter materials

and, "have smaller engine compart-

ments-more cargo space, per

unit of hors~ower.

Because highway regulations on

weight are strict, but pay-load has to

be greater to cover costs of opera-tion.

And instead of just a few types of

heavy equipment, diesels today power

a wide variety of smaller, lighter con-

struction, farming, mining and marine

units.

And some of them demand a lot

of different things of the engine.

Driver conditions are different, too.

Routes are longer and two-man,

sleeper operations often mean days

in the cab instead of just hours.

The traffic worries are greater, so

that any driver-irritation problem is

bad for highway safety.

And research has found vibration

to be highly distracting and irritating

to humans.

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4

Cost of downtime has certainly

changed -which means that any-

thing leading to engine or vehicledamage is expensive, because -

Competition is keen, and customer

satisfaction is important

Equipment is expensive, and in-

operative equipment is more so.

Repair labor rates are high, and so

are replacement parts.

So engine people -whether they

design and build them, or install them,

or repair them, or use them -have

to be concerned about vibration, as

an engine and vehicle problem.

As an engine and vehicle problem,

then, -WHAT IS VIBRATION?

There are two types of engine vi-

bration and we'll take a look at

LINEAR vibration first.

"Linear" refers to the imaginary

lines through the engine, around

which experience has taught us that

the engine will move during its normal

operation.

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engine moves around these

because of engine torque re-

way the engine body, or mass,

to combustion pressure, re-

piston action, and the

of the various shafts and

the engine

from a hoist by just one

one point from which the en-

package would hang absolutely

and still, is its center of

would have a Itttle different

of gravity, which, of course,

upon the way the weight is

though, the center of

is just above the crankshaft,

the center, lengthwise.

we could then start the hanging

cause it to move slightly

side to side, forward to back,

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6

Now, to this already vibrating en-

gine we have to add some parts not

absolutely necessary to get it to run.

Just parts absolutely necessary to

make it do the work its supposed to

do.

Like a generator, and air com-

pressor, and alternator, and maybe a

turbo and some others. Accessories.

And a lot of them have movement to

add.

Many of them can be highty bal-

anced, so that they run smoothly and

add little roughness to overall engine

movement.

You've seen balancing marks like

these- balancing is done by adding

or removing weight. Just about the

way the man balances your auto-

mobile wheels; and for about the

same reason.

Because if we added an accessory

to our hanging engine, and the ac-

cessory was operating out-of-balance,

it would try to push the engine someway or other.

Just about the way automobile

wheels behave when they are out-of-

balance.

But in order-for an engine! to do the

work it's supposed to do, it ~as to be

more or less firmly attached to a

driven unit 01 some kind.

Now what will happen to all the

movement?

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7

Well, that will depend. ..

If the engine is solidly fastened

down -in an extremely heavy piece

of equipment or on a firm stationary

base -the weight and mass of the

engine block {it's still cast iron), plus

the weight of the driven unit, will con-

trol a great deal of the normal move-

ment.

But, for reasons we'll go into a

little later, accessory problems may

develop> Because engine torque re-

action may be transmitted to other

parts of the assembly in some way.

If the engine is attached to a lighter

weight unit, be it off-highway or high-

way vehicle, movement can be trans-

mitted to the frame in which the

engine is fastened.

And sometimes even normal oper-

ating movement can be a disadvan-

tage to the vehicle or the operator .

Not to mention what may happen

when out-of-balance operation (due to

wea~ or poor maintenance) is present.

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8

The builder of the engine, who

knows its center of gravity, the move-

ment around each axis, and the oper-

ation of the accessories it will need,

uses this information to figure out the

best kind of engine moun.ts for each

type of engine and frame arrange-

ment.

In the heavy, solid type of frame-

work, heavy-duty, solid mounting may

be alright- if special care is used in

attaching engine accessories.

In the lighter-weight frames or on

less solid stationary bases, it is usu,.

ally best to use "soft" mounting sys-

tems. These are combinations ofrubber "bumper" units with metal re-

tainers.

And each type of engine/frame

combination can change the shape

and type of ideal mounting system.

An ideal system is one which allowsthe engine to operate efficiently, and

even a little inefficiently, without

transmitting enough movement to the

framework its in to damage any part

of the assembly.

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9

The engine manufacturer, then, can

usually recommend to the manu-

facturer of the vehicle or the stationary

installation an ideal mounting sys-

tem for the conditions.

But an ideal mounting system isn't

always used.

For this reason, as you may imag-

ine, some engine installations trans-

mit more vibration to the assembly

they're in than do others.

In addition, poor maintenance can

keep even a good mounting system

from isolating engine movement tot-

ally.Like fasteners left out or loosened.

Worn or deteriorated rubber that isn't

replaced. Cracked or broken brackets

or crossmembers. Out-of-balance en-

gine parts or accessories.

..

Any type of rough engine operation

also affects balance and movement.

like exceeded wear limits and

bearing clearances. Belts too tight,

or misaligned shafting and gears. Use

of the wrong part in a balanced as-

sembly. Bad timing, detonation, mis-

firing.

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10

Overall vehicle and engine condi-

tion, then, along with good mounting

and isolation determine how much

normal engine movement is magnified

and transmitted.

However, quite often, the attach-

ment of individual parts and acces-

sories can greatly influence the effect

of normal or excessive vibration.

Most of the engine components are

attached by the engine manufacturer,

and you may have noticed that they

are generally pretty close in.

This helps protect them from any

excessive movement from the engine

or from the vehicle frame.

Movement near the engine's center

of gravity is very much controlled by

its own weight, and by the engine

m-ounts. Usually there isn't muchmovement near the center of gravity.

But ~ you move out away from thatpoint, movement increases and force

of movement also increases.

Remember how a game of "crack

the whip" went?

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11

Any component or accessory at-

tached to an engine must be as near

the center of gravity as possible so

there is less of this. ..

And more of this. ..

Sometimes accessories are at-

tached where they will be easier to

service, or to allow a certain shape of

sheet metal, or for some other con-sideration.

When they are, extra care as to

type of bracket and maintenance they

get will be necessary if they have ade-

quate service life.

Remember our hanging engine andthe out-of-bafance accessory?

When the engine is fastened down,

the force that moved the hanging en-gine cannot move the r(gid engine -

it destroys the part or its bracket, or

both.

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12

The same whip action we men-

tioned just now is the source of

another type of engine damage re-

lated to linear vibration -but not

engiile-orginated vibration.

Many things the vehicle or frame-

work may encounter during operation

can cause engine and accessory,

damage. This for instance. ..

Or something like dropping a

loaded trailer on the dolly, ...

Or dropping a six-ton rock into the

dump bed.

And there are all types of drive-

line unbalances and misalignments

which affect engine operation.

Something like misaligned shaftingor run-out (wobble) in a coupling

flange. can fling parts, a fan for in-

stance, off the engine.

.When driveline shafting is unusu-

ally long, or complex, a set of angles

which are uneven or joints which are

not parallel can cause linear vibration

which may break the shafting near

the center (or burn out the needle

bearings in couplings because of lu-

brication starvation).

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13

Stationary installations have unbal-

ances or misalignments, also, which

may increase vibration.

In a generator set, for instance,

bearing wear or shaft misalignment

may lead to uneven air gap inside the

generator housing and cause exces-

sive vibration of the complete as-

sembly.

Incorrect wiring from the panel to

the coils in a generator set also in-

duces excessive assembly vibration

at start-up.

(Thirty seconds of operation under

these circumstances can burn up the

generator .)

In large hydraulic or pneumatic

pumping units, excessive restrictions

in piping sometimes create vibration

for the unit.

Misaligned shafting and uneven

bases also induce vibration.

All these problems lead to linear

vibration, "shaking," that is -and al-

most all of them can be spotted by

a sharp-eyed Technician looking over

an installation.

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But there's one origin of linear vi-

bration which is a lot sneakier, and

harder to locate; but just as destruc-

tive.

And that's resonance.

Resonance can develop and be-

come noticeable and damaging even

if all parts of the engine and driven-

system are operating smoothly within

themselves.

The conditions which determine

whether resonance may become a

factor in the assembly's operation are

the frequencies at which the engine

and components of the system vibrate.

The engine, as designed, moves

about those lines we mentioned due

to its inherent engine torque reaction.This movement has a frequency which

varies with the engine and its RPM.

As the drawing illustrates, .fre-

quency refers to the number of times

the engine mass moves back and forth

across a I ne of rest.

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15

Frequency is measured in cycles',

and a cycle consists of the time it

takes for the movement to go one di-

rection from the line of rest to a stop,

return across the line to a stop in the

()ther direction, then return to the

line of rest.

Normal engine operation causes

the engine itself to vibrate at a cer-

tain frequency -move back and forth

at a certain speed.

Engine parts, engine accessories,and vehicle parts and accessories also

vibrate at some frequency, depending

upon their own operation, their weight

in r~ation to the engine and vehicle,

and theirtocation in the assembly.

All goes well if, at normal operating

speeds, each one is vibrating at a

different frequency.

Howeve:, ..en some other vibra-

ting mass settles into a natural oper-

ating frequency identical to the

engine's operating frequency (samenumber of cycles per minute) at a

speed where lengthy operation is

likely, vibration at that speed in-

creases drastically.

They team up, so to speak, and

like a spring compressed and re-

leased too rapidly, go a little .'wild".

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16

Resonance is generally indicated

by roughness of operation which is

noted at one speed, but disappears

when the engine is speeded up, or

when operati~n is slowed down.

It may escape notice, however, until

the resonating part is found to be op-

erating incorrectly, destroys itself, or

falls off due to a broken bracket.

If resonance is suspected, it is usu-

ally best to have the unit checked on

vibration analysis equipment by anexperienced operator .

Repeated failure of the same ac-

cessory or bracket often indicates

resonance even if the vibration can-

not be detected. Correction of this

condition requires:

Using a rubber mounting of some

type, or beefing up the bracket.

Changing the operating speed of

the accessory.

Changing the weight, the metal, or

shape of the part.

50 far we've discussed LINEAR VI-

BRATION.

It can be seen and heard and felt.

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other type of vibration which

combustion engine

s TORSIONAL VIBRATION.

word torsion comes from the

old Latin word that torque came

-torquere -which meant "to

a good way to refer to

condition it describes.

the word "vibration" is a little

when used for this con-

use of the word "stress" might

a little better because the two

movement are quite different.

torsion present in an internal

engine is present only in

crankshaft and driveline.

cannot be seen cannot be h~d

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It develops because the. recip

cating action of the piston and

assembly transmits "twisting" mo

to the crankshaft and driveline.

Since the cylinders fire one a

time, this twisting force is transmitt

to the crank in cycles, or waves,

torque.

These cycles of torque or "twi

travel out through the crankshaft

driveline to perform the work of

engine, but are not distinguishable

human sight, heari,ng, or touch:

-

The cycles are much like so

waves or water waves. As they

ceed through the shafts, intense p

sure is followed by less pressure;

the next cylinder fires pressure

comes intense again.

IHowever, instead of encounteri

unresisting air or water, these wa

are traveling through necessari

dense and rigid metal.

During normal operation, the bu

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The "vibration" damper has just

one function -to dampen (reduC-eT

the twisting stress which tends to

build up in the rotating crank and

driveshaft during engine operation.

In most highway vehicles, just the

proper flywheel arid damper, in good

condition, and properly installed are

all that is necessary to protect the

engine from torsion.

However, in off-highway automotive

vehicles and in stationary engine in-

stallations, conditions often en-

countered in the driven unit may per-

mit development of unusual, exces-

sive, and even damaging torsional

stress.

Excessive resistance- enough tohold the shaft at the low pressure part

of the cycle until the next wave of

high pressure overtakes it- is most

likely to be found in one of the follow-

ing conditions.

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20

I-Massive Rotating Units Driven- by

the Engine

.Generator Sets

.High Capacity Pumps

.Massive Gear Drives

"-Medium to Large Rotating Masses

When They are Remote Mounted:

.Remote Mounted Torque

Converter and Transmissions

.Large Air Compressor Units

.Cable and Boom DriveCombinations on Cranes, etc.

III-Driven Units Coupled to the Front

of the Engine

.Winches and Reels

.Large, Remote Fans (Engine

Cooling System)

.Hydraulic Pumps

.Cement Mixer Drives

Of course, special damping, proper

mounting arrangements, and flexible

coupling where possible will protect

such a system from excessive tor-

sional stress.

This usually requires that engine

people work with the builder of the

rest of the assembly to get the pro-

tection built in.

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21

In any of these installations the

specified damper and damper mount-

ing parts, overall smooth engine op-

eration, and regular maintenance are

especially important.

The next question is- "Is it ever

possible to make an educated guess

as to whether torsional stress may be

the cause of problems or failures?"

The answer is "Yes" -there are

sometimes indications that torsional

stress is present, and some failures

which indicate it as their origin.

(Some of them also indicate pos-sible unbalance of other types -

there are very few "sure things" in

troubleshooting, you know -and in

a complex system many things may

need checking out.)

Minor Symptoms

.Belt slap in properly adjusted drive

belts.

.Driveline rattle at certain speeds.

.Noisy anti-friction bearings (needle

bearings) in the driveline.

.Extreme heating of the damper after

only -Short periods of operation at

engine speed.

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22

Dri\'eline Failures

.Fatigue fracture of driveline, (near

the center of shafting or in the

splined section)

.Weld fracture in the driveline tube

.Coupling or flange fracture

.Beaten out splines

.Broken synchronizer

Engine Failures

.Crankshaft fracture at the nose or

near the #4 journal

.Gear teeth (pitting on both sides)

.Vibration damper alignment slip-

page (not necessarily excessive tor-sion when at high mileage)

.Accessory driveshaft failure

.Flywheel capscrew fretting/failure

.Failure of hubs at front of engine

Just a word of caution, now, which

may save you some time:

Almost all of these symptoms and

failures may stem from an inoperative

damper .

While a damper does not reduce

linear vibration, it certainly can add

to that (as well as allow excessive

torsionals) if it is untuned, out-of-bal-

ance, or in any way defective.

The damper is a very sensitive part

of the engine system.

PROPER HANDLING TO PREVENT

ITS BEING DAMAGED -AND USE

OF THE SPECIFIED DAMPER AND

MOUNTING PARTS CANNOT BE

OVER EMPHASIZED.

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Any of the following conditions can

lead to an inoperative viscous type

damper:

.Loss or hardening of fluid

.Cracks, nicks or fretting of any por-

tion of the metal

.Damage to an outer surface which

strikes the inertia member.

.Wrong damper, defective material

or high mileage.

The following conditions can lead

to an inoperative rubber damper .

.Nicks, cuts, or deterioration of the

rubber

.Damage to any of the metal parts

or attachments

.Slippage -evidenced by unaligned

indicator marks

.Wrong damper, defective material

or high mileage

DAMPERS CANNOT BE REPAIRED

IN THE FIELD. THEY MUST BE RE-

PLACED IF THEIR CONDITION IS IN

DOUBT .

(See the Engine Manual for

correct checking methods)

Careful and knowledgeable exami-

nation of a unit may locate a mechani-cal source of excessive linear or

torsional vibration. If correctiqn of an

apparent source does not eliminate

the problem it may be necessary to

ask for analysis of the unit by profes-

sional analysts with elaborate testing

and recording equipment.

OUTERHOUSING

WEIGHT

SILtCONE FLUID

HUB

RUBBERMEMBER

INERTIAMEMBER

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Service Division

Cummins Engine Company, Inc.

Columbus, Indiana, U.S.A.

47201

Bulletin AX1001530 >pyright 2-71

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985601 Vibration