Sunday, Sep 07, 2008 at 19:07
Hi Curious.
The oil was held in a large receiver, often on the outside wall of the garage. This receiver is piped to the bottom of the hoist cylinder. It was necessary to bleed all the air out of the pipe work and hoist cylinder (just like brakes on a car) to ensure smooth operation.
To lift the hoist one simply turned on a tap that fed air from the garage compressor into the top of the oil receiver (air over oil). This drove the oil out of the receiver into the hoist cylinder and the hoist rose. To lower the hoist one shut off the tap from the compressor and opened another one to exhaust the air out of the receiver to atmosphere. The oil pipe should always have restrictor fitted to regulate the rise and fall rate of the hoist.
Almost invariably the hoist cylinder was of a displacement cylinder type. This means that it had no piston as such and was sealed at the top of the cylinder where the hoist shaft came out of the outer casing. The shaft itself is the piston and the lifting power is a function of the end area of that shaft x air pressure from the compressor.
They are a very successful hoist, though they had one very nasty habit as they got a bit old. The gland bearings at the top of the outer casing would wear and allow the hoist to tip over slightly when at the top of its stroke. It would then jam there when the mechanic tried to lower it. If he then left the down tap open, then walked over and gave the thing a shake to free it up and the hoist would go into free fall, the speed of which depended on whether the restrictor had been interfered with, and it often had. I think this is what caused their decline in popularity.
Sorry for the long dribble and hope I haven’t confused you to much.
CliveG NZ.
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