Theoretical service life

Serial hoists are not built for unlimited running time but for a certain number of operating hours at a certain loading. This design basis is expressed by the mechanism group under FEM 9.511 – 1Bm, 1Am, 2m, 3m, 4m. Each group has a theoretical service life D in full-load hours.

FEM 9.755 describes how the operator tracks the portion used up. As long as actual use stays below the theoretical figure, the hoist is working within its safe working period, S.W.P. for short. Once it is reached, a general overhaul is due.

Actual use S results from the running time of the hoist T and the load spectrum factor km: S = km · T. The factor rates how heavily the hoist was loaded on average. FEM distinguishes four spectra: light with km = 0.125, medium with 0.25, heavy with 0.5 and very heavy with 1.0.

A hoist that mostly runs with small partial loads therefore uses up its service life eight times more slowly than one constantly lifting its rated load. If running time and loads are estimated rather than measured, FEM 9.755 provides for a factor that covers the uncertainty of the recording method; the value is given in the operating manual.

A chain hoist in mechanism group 2m has a theoretical service life of 1600 full-load hours. Assume the hoist runs for one and a half hours on 220 working days, i.e. 330 hours a year, with a medium load spectrum. Annual use is then 0.25 · 330 = 82.5 full-load hours.

Without the recording factor, the service life would be reached after a good 19 years. With a heavy spectrum and the same running time, that period halves; in two-shift operation it halves again. What counts is always the running time of the hoist motor, not the length of the shift.

Most accurate is a data logger in the hoist that records running time and load itself. The next best solution is an operating hours counter combined with a reasoned assessment of the load spectrum. Without a counter, both quantities can only be estimated from the work process – with the largest factor.

The portion used up is determined and documented at the periodic inspection, as a rule once a year. If there are no records at all, a remaining service life cannot be demonstrated.

Once the theoretical service life is used up, the hoist may only continue in service after a general overhaul. The fatigue-loaded parts of gearbox, brake and chain or rope drive are checked and replaced as specified by the manufacturer. A new safe working period then begins, its length set by whoever carries out the overhaul.