AVOIDING FATIGUE FAILURES

 

Manipulator fatigue

 

Manipulators on a tough duty in a forge or foundry may be doing a cycle every 6 seconds and for 24 hours per day - that is more than 7 times the number of cycles that might be performed by a standard hydraulic manipulator. The result is an unacceptable number of failures in competitors' manipulator structures after quite short operational periods.


 

Clansman Dynamics has recognised the problem and produced a design which allows for a cycle every 6 seconds and a 10 year life.


 

Clansman Dynamics Solution

The first step was to develop a computer programme which would calculate stresses for all positions of the manipulator with no assumptions.


 

Nobody should trust such a computer programme without verifying with a scale model and then strain gauging of the prototype machine. These were the second and third steps taken by Clansman Dynamics engineers after the machine had been designed to the correct standards.


 

One competitor uses PTFE spherical bushes.


 

Another competitor uses a mixture of needle roller bearings, roller bearings and steel spherical bushes.


 

It is worth noting that one large Italian foundry which has destroyed a machine from one competitor and which has 4 limping machines from another has made a campaign change to replace spherical steel bushes not with PTFE spherical bushes which would have been a straight swap but has machined new parts with larger bores for roller bearings.


 

There is no denying that manipulators present a severe test of any bearing because of shock loadings, dirty environment and the fact there is not much more than a 90 degree movement on any joint.

 

 

 

Precise knowledge of stress levels is critical

International standards are available for the design of structures which will not fail. These standards describe the types of loading, materials, permitted stress levels for a certain life, etc.

 

One line of permitted stress levels taken from a typical table shows the nature of the problem:


 

Number of cycles (millions)

0.125

5

10

20

40

Years of operation

0.03

1.25

2.5

5

10

Permissible fatigue stress N/mm2

221

156

143

131

120

Permissible fatigue stress Tons/in2

15

10.6

9.7

8.9

8.16
 

In simple terms this example shows that if the complex stress calculations are wrong by 10% then the life of the structure may be reduced from 40 million cycles to 20 million cycles or worse.

 

The resulting manipulator

 

It is useless to achieve the correct stress criteria by simply adding weight. Instead it is necessary to use steel sections with a larger modulus, steel castings where weld fatigue problems are especially difficult, weld and profile details which avoid stress concentrations, bearings which are correctly sized and whose life calculations have been checked by a leading bearing manufacturer such as SKF or FAG.

 

Above all, knowledge is required. Check this out with our competitors. Fatigue avoidance is vital but only a start: shock and stall loads must also be taken into account and their effects engineered out.