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The other day I had a call from an elevator
technician who had responded to an elevator trouble-call
(commonly referred to as a "callback") and found the oil level was low in the reservoir.
His supervision is not elevator people - they are managers - and he wasn't sure about how to proceed, so hence the call. We'll get back to his situation
shortly.
Most hydraulic elevators have an
internal timer that begins a preset cycle of time when the elevator begins an
up run. If the elevator fails to reach the designated landing in that time
value, the timer circuit activates and shuts off the pump. This code-required
function is often called the Low Oil Timer or LOT. It's now required to be
tested annually in most states. Other names might include Stall-Timer or
Trip Timer.
Of course, a low oil condition is only one of the causes of an
elevator not completing it's run in time. Mechanical binding of the car is
uncommon but happens. Faulty circuit or valve coils, the hydraulic valve
itself, or a closed shut-off valve will also do it. On most systems once the LOT is
activated the elevator returns to the bottom landing. If it's at any other landing
when the timer expires, the doors open to allow passengers to exit. Then the
elevator is out-of-service until a technician responds and resets the LOT.
LOTs are not auto-reset devices for a very good reason: safety. Why did the
LOT activate? If the problem is low oil, then what's the cause - and where is the oil?
Before we explore the why and
how, we need to examine the elevator equipment we are dealing with. A simple
1980s vintage hydraulic freight elevator, 3 landings served, 34' of total
rise. Traditional 4' pit, single in-ground jack assembly positioned at center
of the car. In these units the jack goes in the ground as far as you go up;
single stage piston, 8" diameter, 10" jack. There is a 3" oil line between the
single pump unit to the jack that is exposed and easily examined. Much easier
than oil lines that are buried.
Hydraulic jacks used for elevators manufactured
after 1972 have a double bulkhead in the bottom. See my drawing below. If the jack
corrodes at the bottom, oil leaks out but at a slow rate; the elevator lowers
due to gravity at a slow speed. Should a major hole happen, these things drop
like a rock and sometimes people die. It's tragic, but it does happen.
Imagine crossing the
threshold and the car drops out from under you. That's why we take oil leaks
seriously. The bottom of the jack is the only area that has extra protection;
jack walls are single layer. As an additional protective measure, some areas
require that the jack assembly be encased in PVC with a sealed end cap to prevent
corrosion. This also offers nifty ways to pressure check for oil leaks. We'll
go into that in another article.
In all the incidents I've looked at and have
been involved with as an elevator mechanic and service manager, there has
always been a record of oil usage prior to the failure. It's what had happened
in response to that data that made the difference between filling out a repair order
versus an accident report. Code requires that an oil usage log be maintained and all
oil be accounted for. This doesn't happen more than it happen. I've been on sites
where no log is kept or the reverse; the oil log is kept and we've seen a steady
addition of oil and no further investigation of where the oil goes. Failure
results and we are left with the aftermath.
The LOT worked in this case; the
elevator was off when he arrived. A quick look revealed no oil leaking form the
reservoir, no oil evidence at any point of oil line from the pump unit to the
jack in the pit, and no oil in the pit. That leaves only the jack assembly in
the ground.
Now we get to his call;
what's the best way foreword? The elevator held enough pressure to allow upward
movement, so before he called he had placed supports on the guide rail under
the raised elevator so the car was resting on these installed supports to allow
access to the pit.
My standard is to place suitable pipe supports under the
elevator between the buffer stands and strike plates if it is safe to do so.
What I've done is raise the elevator with chain hoist placed above the car.
Safety first. Why get under a potential falling weight? Then we can look at the
jack assembly - if only it weren't underground. Only a short section
containing the packing (seal between the piston and jack) is exposed and that
part rarely is suspect in these cases. If oil was present near the jack on the
pit floor, that's a different story. There are test procedures to follow and I
related those to my mechanic friend. More on the results in the next post.
- Joe
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