Dynamic impulse and shock loads
Dynamic impulse or shock loads occur when high-velocity movement is followed by a sudden abrupt stop.
For forklift truck leaf chain, this type of movement can impose exceptionally high loads which exceed the fatigue endurance limit.
An adverse movement may result from carrying a suspended load over an uneven surface such as tracks, potholes or rough ground or when trying to 'inch' loads beyond the rated capacity of the leaf chain or lifting mechanism.
Continual operation of leaf chain that results in “bounce”, under otherwise normal and apparently safe operation, will cause cyclic (impulse) loading, eventually leading to fatigue failure.
This cyclic loading on the leaf chain is intensified when the operation is running at high speed (when the actual load on the leaf chain will be roughly doubled.)
If a leaf chain is repeatedly loaded above its fatigue endurance limit, it is likely to experience premature fatigue cracking.
It is not always easy to establish the fatigue endurance limit of forklift truck leaf chain, but it will usually be lower than its certified SWL.
Typically, forklift truck leaf chains will become worn (elongated) well before fatigue sets in and so it is only when a chain is subjected to continuous impulse loading that fatigue failure occurs.
Fatigue cracks (which are not necessarily visible) are caused by periodic on-off loading above the endurance limit of the leaf chain.
The factors that determine when fatigue failure will occur include:
- the magnitude of the load
- the frequency of its occurrence
- the endurance limit of the forklift truck chain
These sorts of cycles and environments can make it difficult to predict forklift truck leaf chain life so it is essential to conduct regular inspections to evaluate leaf chain replacement life.
Effective lubrication is a key factor in the longer service life of leaf chain.
It is common for forklift truck chain users to assume that their leaf chain has been adequately lubricated. But nevertheless, friction corrosion, turned pins, metallic friction, stiff joints, noisy operation and pitting still occurs.
This is especially the case when the applied lubricant adheres only to the exterior of the leaf chain and doesn’t penetrate the leaf chain joints.
The life of an adequately lubricated forklift truck chain is estimated to be 60 times longer than that of a dry running leaf chain. And even temporary dry running will shorten leaf chain life considerably.
Selecting the correct lubricant for forklift truck leaf chain can make a huge difference.
Most chain lubricants for example are designed for transmission or motorcycle applications, whereas the loading and operations of leaf chain require a lubricant with totally different properties.
The graph above shows the results of a test which simulated the work a leaf chain would do when fitted on a forklift truck mast.
These leaf chains were loaded to near their maximum working load and repeatedly raised and lowered. In this test three identical lengths of leaf chain were used - and each applied with a different type of oil.
At 40,000 cycles, the difference between the best performing leaf chain and the worst was an amazing 3.8 times.
At this point, the A sample seized and would no longer articulate. The two other leaf chains continued for a further 40,000 cycles, double the amount compared to oil A.
Another important factor to consider is keeping the leaf chains constantly lubricated. Forklift truck chains that operate outdoors, that are in environments where they are regularly washed or that work within cold stores (where they are subject to condensation) will require an oil that will stay in place.
Under these circumstances, it is unlikely that operators will re-oil their forklift chains regularly enough to achieve the leaf chain’s optimum life.
In another leaf chain study, the same lubricant was applied to three identical samples of leaf chain, which were then rinsed with water for one minute and then subjected to a comprehensive salt spray test where a hot salt water mist was blown over the leaf chain at regular intervals.
Rinsing the leaf chain with water beforehand simulated normal usages, such as when a forklift truck chain is used in outdoor applications or cold stores.
This accelerated corrosion test was able to assess the corrosion resistance of the leaf chain without having to leave it outside for months, or years, on end.
Oil A after 40 hours in salt spray test
Oil B after 40 hours in salt spray test
Oil C after 15 hours in salt spray test
The image above shows that the leaf chain lubricated with oil C performed well in the wear test, but did not perform at all well in the corrosion test. This was due to its lower viscosity, which helped the oil to penetrate between the pin and plates, but which also made it more easily removed by washing.
For forklift truck chain working outside or in cold stores, the oil would not stay in place long enough to perform its main function of lubricating the forklift truck leaf chain. Oil B, however, remained in situ longer on the leaf chain and therefore offered the best mix of wear and corrosion resistance.
Many forklift chains get encased in dirt and dust, which prevents oil from flowing to the important load bearing regions of leaf chain between articulating link plates and pins.
It is vitally important that all leaf chains are kept free of dirt and debris to ensure effective lubrication of the critical contact areas.