Home Forums General Discussion Technical Questions Why use breaks on geared traction machines

Oh we do know why mate think about it. If you disconnected the worm gear from the brake drum, with the car still attached via the ropes, unless the car/cwt are balanced perfectly in the shaft the car will roll up because the cwt is generally 40 to 50% heavier by design. if the ropes were removed from the traction sheave then there is no load and therefore the no movement on the sheave/crownwheel/worm gear.

Hope this clears it up for you.

THIS is how we know your theory doesn't work.

You only have to ask any lift engineer, they will confirm if the car is empty when you release the brake the car will start to move up, simples!

we all know why, it was made very clear, during our lengthy apprenticeship, we could let you in on the secret, but since you have never worked in the business it would be a waste of a good secret.

This question has to be a gee up (Australian slang for joke).
Who is RCD?
Maybe an apprentice or someone who works or studies near elevators.
Some people have suggested you try disconecting brake.
My advise don't do it.
RCD do you think anybody would waste money on anything that isn't absolutely necessary?
RCD if you are in the lift industry eventually you will see a lift car belonging to a geared traction machine crashed up into top of shaft or fully loaded crashed in pit that the holding brake failed and the penny will drop.
Good luck and stay safe lifts are killers hope you make it to retirement age alive there is a reason why 7% of a lifties pay is danger money.

I sorta see what he is asking. If the worm and ring gear mesh were truely 90 degrees from each other-you would have a "locked" gear. The gears are not 90 degrees-they are angled and swept in relation to each other. This allows a very smooth mesh that does not transmit noise.

This is from machinedesign.com

All theoretical analyses of self-locking worm gears deal with static conditions. In such an analysis, the load on the worm gear can�t drive the worm if the coefficient of friction between worm gear and worm is larger than the tangent of the worm�s lead angle. In other words, the friction angle must be larger than the lead angle to prevent backdriving.

Within the last 3 years, our failure analysis engineers investigated two accidents involving worm gears where the user was injured. In each case, a designer selected a worm gear speed reducer from a catalog and incorporated it into a lift device intended to raise and lower people. In both cases, the designer was under the impression that the load (person plus platform) could not self-lower. They both relied on self-locking worm gears to hold the cargo in a desired position and genuinely believed that no brake was necessary to supplement the worm gear in such a critical application.

So it is a matter of friction. There is your answer!

Let's take a look at this winch in this youtube video [link below](way to small to lift an elevator but still the same principle). If I were to use this winch in a dumbwaiter to lift a load of 100lbs (no counterweight / lifting straight up) are you saying that the 100lbs load would turn the input (the worm) and descend?

https://www.youtube.com/watch?v=LUYqVE1R2KA

Theory and reality are two different things. You can't tell untill you try it.

There are safety codes written because theory doesn't equal safety.

Firstly I would assume that when it states its 'self locking' (depending on any 'locking feature' on the worm gear ie caravan winders that have that clicking sound which is a reverse lock), I would imagine it has a load maximum and that it depends on runaway characteristics as well ie if the load is down hill. So if the winch is over loaded then it can runaway. Remember to that a dumbwaiter is also an 'old style' hoist ie a drum drive system which also has a braking system either a drum brake on the old units or a disc brake on the new units like a overhead crane motor.

RCD, I think your forgetting that elevator Ring and worm gears have a lead angle to improve efficiency. The sample you've given has no lead angle and cannot be compared to an elevator machine. As a rule of thumb, any Ring-Worm Assembly with a lead angle more than 5 degrees has the ability to Back Drive and must have a brake for safety. If you have the time and inclination just Google WORM GEAR BACK DRIVING and you will have all you need to know.

If they are geared low enough, they won't overhaul. We modernized two old Circle Bar W cars, 200fpm and 1750 rpm motors and they wouldn't move with 125% load. Even if you ran the car down and cut it off with the brake picked. I won't say it didn't drift a ways, but it came to a stop before checking the spring buffers.

WHEN I WAS FIRST IN THE TRADE I HAD A MECHANIC READ THE PERMIT IN THE ELEVATOR AND NOT THE CROSSHEAD CAPACITY. CAR WAS 2000LBS HE HAD ME PUT IN 3000. The worm, and the brakes didn't hold the weight. Car crashed into the pit. You need brakes TRUST US!