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Elevator and Escalator Consulting Engineers

Preventive Maintenance?

The concept of "Preventive Maintenance" has long been accepted in the elevator/escalator industry as an article of faith.

The idea is that by carrying out certain routine maintenance checks and procedures it is possible to reduce the incidence of call-backs and unscheduled shutdowns.

To quite an extent the idea of regular checks proceeded from the need to routinely oil and grease various parts of the elevator in the era prior to sealed bearings. Elevators in the first half of this century had a number of large bearings that required periodic greasing and checking.

There is very little need for regular lubrication on a modern elevator and the necessity for routine checking has been minimized by the use of more non-wearing parts. At one time the maintenance mechanic had to check controller contacts and clean them or replace them on a fairly regular basis. Today these contacts have to a large extent been replaced by solid state devices.

Of course each visit by an elevator mechanic to the site involves traveling time. In many cases the traveling time will exceed the time spent working on the elevator.

For all of these reasons the routine maintenance procedures have been called into question. More and more the major elevator companies are moving away from fixed time based maintenance. They are attempting to establish by means of historical data the life expectancy of components. With this information the possibility of "predictive" maintenance can be considered.

If the particular time at which a component will fail can be established then it is possible to replace it just prior to failure rather than replace it after failure.

Obviously this has advantages in reducing maintenance cost and increasing equipment reliability.

One of the difficulties with machinery reliability assessment - which is the first requisite of a predictive maintenance program - is the rather wide range of life expectancy of a given component.

It is apparent that the mean time between failure for any part is a function primarily of its actual use time rather than just calendar time. The common example is that of elevator lift cables. The life of these cables is generally considered to be proportional to the "mileage" on them. For this reason it makes sense to install a meter to record the running time or distance. Again, the life span of many components is directly related to the number of times an elevator starts and stops; thus it makes sense to install "trip" meters. The inputs from devices such as these then allow the maintenance contractor to determine when to replace components.

In the case of the elevator lift ropes, it is of course true that the mechanic could also check the ropes regularly foot by foot. This is a laborious procedure. In some cases the cost of checking ropes starts to approach the cost of replacing them. In cases such as this the advantage of predictive maintenance is quite apparent.

How effective are the various approaches to predictive maintenance? Too early to tell as yet. The jury is still out.

In any case, it is certain that the current trend towards predictive maintenance will continue.