Testing of Elements and Assemblies

MMM4035L

Laboratory Handout

Loads acting on crane structure during travel

by

Eugeniusz Grabowski, PhD, MSc. (Mech. Eng.)

Wrocław 2009

All rights reserved

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1. Introduction

Loads acting on the structure of cranes vary substantially, depending on their source,

magnitude and type, frequency and duration, which has been reflected in their classification

according to international and european standards covering crane design.

The main sources of loads applied to the structures of the cranes are:

- masses of elements and parts of these structures (including those which either change or

not change their position relatively to other elements and parts),

- masses of payloads,

- accelerations and decelerations in the periods of unsteady work movements of cranes,

- dynamic reactions due to crane’s own masses and carried payloads,

- dynamical reactions of crane runway rails and buffers,

- elastic deformations crane structures caused by deviations in manufacturing tolerances

and differences in characteristics of parallel drives,

- wind pressure.

Depending on frequency and duration, loads acting on the structure of of overhead tra-

velling cranes are classified into:

- regular loads, which occur in normal exploitation and arise from gravity, accelerations and

decelerations during start ups and breaking of the crane work movements,

- irregular loads, which also occur in the normal exploitation and arise from wind pressure

(in seviere wind condition), temperature effects or by the so-called crane skewing – a phe-

nomenon occurring during the crane travel,

- exceptional loads, which occur occasionally in conditions other then normal exploitation

and are caused by very high wind pressure (so-called “out of service” wind conditions),

seismic effects, failure of working mechanisms or sudden crane impact on fenders.

Loads acting on crane structures during travel, qualify into all the above categories and

are those whose proper evaluation is especially significant for safe strength design dimension-

ing, particularly for crane structures characterised by large spans (distance between the rails

of the crane runway) in relation to the distance of the crane’s wheels (on the same rail of the

crane runway).

Amongst the loads, which are more important from the point of view of (already men-

tioned) safe strength design/dimensioning of crane structures, and which occur during or as a

consequence of crane travel, it is necessary to distinguish between:

- loads caused by crane skewing on the crane runway,

- loads caused by travelling on an uneven surfaces of the crane runway,

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- loads caused by crane impact into fenders or by impact into another crane on the same

crane runway.

These irregular loads, which all do not occur at the same time, are accounted for in the

design calculations by the so-called co-incidental systems of combined loads, which by acting

simultaneously in vertical and horizontal planes in different parts of the crane structure cause

its effort which is checked by design calculations according to the codes of practice (stan-

dards) ISO or EN.

2. Theoretical and codes of practice bases for defining loads acting on crane structure

during travel – loads caused by crane skewing

Skewing, understood to be a deviation of the crane from its ideal, or intended alignment

(in which the longitudinal axis of the crane bridge is perpendicular to the crane runway axis),

is an external symptom of the crane skewing process during travel. During skewing, the load-

carrying structure of the crane undergoes rotations relative to the crane runway (within the

gaps between the rail and flanges of the crane’s wheels or crane guiding elements) and simul-

taneously experiences elastic deformations in the horizontal plane [1].

External loads acting on the crane structure, in the horizontal plane are balanced by the

forces in contact points of the runway rails and tracks of the crane’s wheels (forces of fric-

tional contact), flanges of these wheels or side guide rolls (forces of rigid contact).

Such a crane structure, treated as a non-deformable frame in the horizontal plane and

moving in a plane , is subjected to longitudinal and transverse (relative to the rail axis) com-

ponents of skewing force Fx1i, Fy1i, Fx2i, Fy2i (tangential forces). These forces arise from the

rotation of the system about the so-called instant slide pole (Fig.1) and are balanced by one,

largest in magnitude, guiding force Fy, occurring at the interface between the rail and just one

element guiding the crane along the runway. It is a case of traveling with one guiding force,

which amongst other cases (traveling with a number of guiding forces) is the most undesirable

from the point of view of the magnitude of horizontal loads that it generates.

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Fig.1 Forces on the load-carrying structure of the crane caused by skewing

The magnitudes of the aforementioned forces are determined according to the standards

[2] and [3], depending on the features of the travel mechanism (with central or individual

drives), the method of wheel attachment (with or without possibility of lateral movement), as

well as the basic dimensional ratios of the crane structure (the crane bridge span to the wheel

distance on the same rail).

The magnitude of the guiding force Fy is determined according to the aforementioned

standards as

Fy = υυυυ⋅⋅⋅⋅ f⋅⋅⋅⋅ m⋅⋅⋅⋅ g (1)

while components of mentioned skewing forces (tangential forces) are given as

Fx1i = ξξξξ1i⋅⋅⋅⋅ f⋅⋅⋅⋅ m⋅⋅⋅⋅ g (2)

Fy1i = υυυυ1ι1ι1ι1ι⋅⋅⋅⋅ f⋅⋅⋅⋅ m⋅⋅⋅⋅ g (3)

Fx2i = ξξξξ2i⋅⋅⋅⋅ f⋅⋅⋅⋅ m⋅⋅⋅⋅ g (4)

Fy2i =υυυυ2ι2ι2ι2ι⋅⋅⋅⋅ f⋅⋅⋅⋅ m⋅⋅⋅⋅ g (5)

where

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f – is a coefficient of rolling friction, dependent on the angle of skewing α, given by:

f= 0,3[1-exp( -250⋅⋅⋅⋅α α α α )] (for α < 0,15 rad) (6)

m⋅⋅⋅⋅ g – the gravitational force due to the mass of the crane,

ξξξξ1i, ξξξξ2i parameters dependent on the location of the instant slide pole (determined by µµµµ’, µµµµ, h

–see fig 1), crane span l, the number n of the crane wheel pairs and type of crane travel drives

(ξξξξ1i =ξξξξ2i=0 for individually powered travel mechanisms),

υυυυ1i, υυυυ2i – parameters dependent on the location of the instant slide pole (µµµµ’, µµµµ, h), the distance

di and the number n of the crane wheel pairs.

3. Methodology of measurement of selected loads acting on crane structure

Measurements of the loads discussed above, which act on real structures, are carried out

mostly by employing strain gauge methods, which, compared to other methods, produce the

most accurate results, provided the measurement procedures described below are followed.

The methodology of such measurements uses one to one relationships (usually capable of be-

ing expressed analytically) between the experimental readings at selected strain gauge points

and the loads acting at those points. Thus strain gauge measurements are indirect methods

and the only useful methods of measurement (with the exception of loads due to the trans-

ported payloads) in situations where installation of appropriate load converters would be im-

possible or would require significant changes to the construction of the crane structure.

Strain gauge measuring points employ rosette strain gauges or single strain gauges con-

nected into Wheatstone’s bridge (whenever possible), ensuring temperature compensation and

signal magnification (even four-fold), compared to the signal obtained from an individual

strain gauge. Both types of strain gauges are oriented along the direction of main stresses

(strains) caused by the measured loads.

The results of the strain gauge measurements, expressed non-dimensionally (usually in

promils ) are linearly dependent on the relative strains εi associated with the normal stresses

σi on a surface of the given structural element (plane stress state), where σσσσi = εεεεiE or

σσσσi = εεεεiE/(1+νννν) (E – Young’s modulus, νννν - Poisson’s ratio). The values of determined in this

way can be used to find the measured values of forces F i, using the relationship F i=F i (σσσσi ),

known form the strength of materials.

The most accurate measurement of loads using the strain gauge method is achieved

through calibration of the measuring points readings in units of forces placed externally at the

exact locations of the measured loads. As a result of the direct experimental measurement of

the relationship between the strain gauge readings and the loads, other effects, such as manu-

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facturing imperfections of the crane structure, variations in strain gauge parameters (resis-

tance, strain gauge constant k) and effects of varying cable lengths (between the strain gauge

points and measuring amplifier) are all compensated for.

4. Example of implementation of strain gauge measurements of selected loads acting on

crane structure during travel

Measured loads (guiding forces) FH1i, FH2i, acting at the points of contact with the rails of

the side rolls guiding the crane along its runway in the laboratory (Fig.2) are, in fact, the reac-

tions to the forces mentioned above, occurring at the time of crane skewing during travel

(more precisely component tangential forces Fy1i, Fy2i, according to Fig. 1).

4.1. Location of strain gauge points for measurement of forces FH1i, FH2i

These points (accordingly to their name) are groups of two pairs of strain gauges bonded

to especially constructed measuring pins in brackets of guiding rolls placed at the ends of both

crane end beams (Fig.2). These pins work in shear caused by the forces SH1i, SH2i due to the

measured forces FH1i, FH2i .

Fig.2 Location of strain gauge points PTH1i, PTH2i for the purpose of measurement of se-

lected horizontal loads FH1i, FH2i acting on the crane structure during travel

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4.2. Strain gauge arrangement for measurement of horizontal forces FH1, FH2i

The system comprises four measuring points PTH11, PTH12, PTH21, PTH22 (pins for measur-

ing of the horizontal loads acting on the crane structure), each of which consists of four active

strain gauges joined into a complete Wheatston’s bridge (Fig. 3)

Fig.3 Strain gauge arrangement for measurement of horizontal loads FH22

All strain gauge points are connected to a multi-channel measuring amplifier. To each of

these points, to one diagonal of its Wheatston’s bridge, voltage [V] (DC or AC - depending on

the type of the measuring amplifier) is supplied with one pair of cables. From the second di-

agonal of the bridge, a measurement power signal is received [mV], which is appropriately

processed in the amplifier ( e.g. amplified and cleaned from interference), and then is written

by the multi-channel register (numerical acquisition of data written into a computer disc).

Fig.4 shows a monitor in the measurement system enabling an observation of the changes of

measured loads. A similar function can be performed by a register using a paper tape.

Fig. 4 Schematic arrangement of strain gauge system for measurement of horizontal loads FH1i, FH2i acting on the crane structure

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4.3. Basic requirements regarding the measurement procedure applied to loads FH1i,

FH2i

The correctness and accuracy of the measurement of loads mentioned above depend,

most of all, on:

- establishing and adopting the appropriate zero level of the measurement signal at strain

gauge points PTH1i, PTH2i ,

- appropriate selection and setting of the measurement range in the amplifier for the individual

measuring channels,

- calibration of the strain gauge readings at measurement points.

In order to satisfy the above conditions, it is necessary to:

1) Determine and set a zero level of the measurement signal from points PTH1i, PTH2i for the

case of completely unloaded side rolls guiding the crane. In order to achieve that, the

crane can be moved short distances in opposite directions so that the successive pairs of

guiding rolls (of the first or second crane end beam) do not come into contact with the

sides of rails of the crane runway. In cases where it is impossible to achieve such an un-

loaded state, the zero level of the measurement signal for points PTH1i, PTH2i can be estab-

lished as an arithmetic mean of at least 60 readings, taken after stopping the crane travel

(in alternative, opposing directions) and releasing crane brakes in 3 different places on the

most used section of the crane runway, or near its ends and centre.

2) Set up the same measurement range on all measurement channels (that is the same range

for FH1i and FH2i) to facilitate later analysis of results. The magnitude of the range should

be slightly larger than the maximum values of the measured forces (then, the need to ampl-

ify the strain gauge signals is smaller, which reduces the effect of external interference).

3) Load each pair of the guiding rolls (those which are not in contact with the rail) with hori-

zontal and perpendicular to the rail force, using at least three different values in order to

check the linearity of strain gauge readings at the measurement points as a function of this

calibrating force. As mentioned in section III it is an activity that ensures the highest poss-

ible accuracy of load measurement possible in the given conditions and given accuracy

class of measuring equipment used in the strain gauge measurement system.

5. Program of the laboratory exercise

1. Safety training.

2. Preparation of the crane and measuring equipment.

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2.1. Determination of zero reading for the individual measurement points (guiding

rolls without contact with the sides of the tops of the rails).

2.2. Determination of the measurement range for the all measurement channels (trial

movements of the crane).

2.3. Calibration of readings of the measurement points ( loading of rolls with horizon-

tal forces of known magnitude).

3. Measurement of forces FH1i and FH2i – registration of strain gauge readings at points

PTH1i and PTH2i

4. Analysis of results

4.1. Determination of the largest guiding force.

4.2. Determination of the calculated value of the leading force according to the stan-

dards [2] or [3].

4.3. Comparative assessment of the values of the measured forces – conclusions

5. Report

ad.1. The convener (leading laboratory exercise) familiarizes all participants with the ba-

sic principles of safety in the area of the working crane and safe conduct of all the

tasks associated with the preparation of the crane and measuring equipment.

ad.2. All tasks are to be carried out in the manner that would ensure satisfaction of the re-

quirements presented in point 4.3. In order to establish adequate level of zero read-

ing of measurement signal from points PTH1i and PTH2i , it is possible to achieve

(other than by methods described in point 1, section 4.3) a complete relaxation of

the guiding rolls through forcing a transverse displacement of the wheels of the un-

loaded crane end beam that is when unloaded hoist trolley stands by the second

crane end beam. In order to establish the same measurement range for the individ-

ual channels, it is necessary to carry out trial runs of the crane over short distances,

in each direction and at full speed.

The largest values registered at the strain gauge points PTH1i and PTH2i sets the

base for the measurement range. During calibration of the individual measurement

points, the largest values of the calibrating force should be so chosen as to ensure

that the corresponding readings at these points were close to the set measurement

range (equal to all channels).

ad.3. In the measurements, depending on the number of the available measurement chan-

nels and possibility of simultaneous recording of their signals, it is necessary to reg-

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ister readings of at least two measurement points located at the same ends of oppo-

site crane end beam.

ad.4. The analysis includes parts (within the scope of work described in p 4.1) which are

to be carried out by the students immediately after completing the measurements of

guiding forces FH1i, FH2i , and the remaining parts (within the scope of work de-

scribed in pp. 4.2, 4.3) carried out by the students individually, after the laboratory

session.

The largest absolute value of the measured loads, FH1i, or FH2i is to be determined

after studying all the changes in the strain gauge recordings at points PTH1i and

PTH2i (noted on the channels of the same measurement range).

The value of the guiding force Fy, calculated according to the standards [2] and [3],

is to be determined from equations(1), (6), (7), (8) using the following data (Fig. 5):

l=5,17 m; ek=1,25 m; er=1,58 m; m⋅⋅⋅⋅ g=85 kN; d1=0,165; d2=1,415 m; n=2;

p=0; µµµµ=0,36; αααα=0.015 rad

h = 21

2122

dd

dd

++

(7)

υυυυ = hn

dd

•+− 21

1 (8)

Fig.5 Scheme of the crane and data for calculating the guiding force according to the

standards [2] and[3].

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When comparing the extreme values of the measured guiding forces FH1i, or FH2i it

is necessary to check the relationship between the direction of crane travel and the

occurrence of the largest guiding force Fy on one of the ends of the crane end

beams (in accordance with Fig.1). Subsequently, it is necessary to compare the

largest value of the measured force FH1i, or FH2i (previously determined in accor-

dance with p. 4.1) with the value of the calculated force Fy (determined in accor-

dance with p. 4.2) and formulate conclusions from all the comparisons.

ad.5. The report on the completed laboratory exercise (in accordance with points 1 - 4)

ought to contain a brief description of the measurements that were carried out, pres-

entation of the results from the measurements and calculations, and conclusions

from the analysis of the results and comparative assessments.

Literature

[1]. Grabowski E: Limiting of cranes skewing. Transport Przemyslowy nr 2/2001

[2]. International Standard ISO 8686-1: Cranes – Design principles for loads and load com-

binations – Part 1: General.

[3] European Standard EN 13001-2: Cranes. General Design – Part 2: Load effects. CEN B-

1050