Ask your worm gearbox supplier if his proposed gearbox can be back driven.

That's the best way ■

Originally Posted by sganesh

Back stops ( also known as hold backs ) are fitted in the gearbox to prevent the inclined conveyors running in reverse when they are stopped with load.

Below what ratio, back stops are required in worm gear boxes?

Is there any guide line?

That would be best answered by the reducer manufacturer. You can also use an external backstop such as a Morse CB series mounted on the drive pulley shaft.

The size is determined by the holding torque required. ■

Originally Posted by Lyle Brown

I think designer may be refering to (in particular refer "direction of rotation"):...

I'm referring to the fact that depending on the characteristics of a worm gearbox, it may not be self-sustaining so may back drive. Just one extra thing an engineer needs to consider when designing a machine drive.■

Originally Posted by sganesh

"Unlike ordinary gear trains, the direction of transmission (input shaft vs output shaft) is not reversible, due to the greater friction involved between the worm and worm-wheel, when a single start (one spiral) worm is used. This can be an advantage when it is desired to eliminate any possibility of the output driving the input. "

Sorry, that a load of total BULLSH1T!!

I for one have direct experience of worm gearboxes being back driven. ■

Originally Posted by sganesh

"Unlike ordinary gear trains, the direction of transmission (input shaft vs output shaft) is not reversible, due to the greater friction involved between the worm and worm-wheel, when a single start (one spiral) worm is used. This can be an advantage when it is desired to eliminate any possibility of the output driving the input. "

Thank you Mr.Lyle. The above was quoted from Wikipedia. I was referring single start ( one spiral ) only. And we are using single start worm only.

I thought requirement of hold backs may MUCH depend on the reduction ratio.

I will ask the worm gearbox suppliers.

Your quite right; ratio does matter. Old Burman worm steering boxes used to exhibit reversibility and some caster angles used to take advantage of that. Reduction ratios were about 12:1. I could get an old eight wheeler back on line by starting to straighten up and then take my hands off the wheel and watch it spin. Some were twin start 15:1. ■

Dear sganesh,

A worm gear is self-locking when the transmission efficiency is less than 50%

The formula for this efficiency is

Worm gear efficiency = Tan(threadangle) / Tan(threadangle+frictionangle)

Conclusion is that a worm gear is not necessarily self-locking, but can be self-locking

A low friction angle compared to the thread angle causes self-locking.

Low thread angles are mostly combined with high transmission ratios, but are not the direct cause for self-locking.

As an example: Lift gears are specially designed to be self-locking. A brake failure will then not lead to a free falling cage (Only some overshoot)

Any good study book will solve the problem, although the consultancy of a specialist is (not always) easier

All for now

Teus ■

Maybe an analogy can explain.

You can drive a wedge underneath a heavy weight.

However, the wedge does not come out between the weight and the floor.

(It is self-locking and the weight, no matter how heavy, can squeeze the wedge away).

If the wedge is very steep and well greased, it just pops out.

A matter of wedge angle and friction angle.

Yes, I think we are talking about the same thing.

Take care

Teus ■

Taken from the Boston Gear web site -

A worm gear set is said to be self-locking, or irreversible when the gear cannot drive the worm. This condition is obtained, if the lead angle of the worm is less than the friction angle, and as a consequence the efficiency for reversed driving is zero. The friction angle for static conditions will vary with such factors as surface finish and lubrication. Based upon the generally accepted value of static coefficient of friction equal to 0.15, the friction angle would be approximately 8°. However, the friction angle decreases rapidly with the start of motion, also, vibrations from nearby sources quite often upset the static condition of a locked set of gearing a sufficient amount to reduce the friction angle to a point where motion occurs, this is sometime called "stiction". These unpredictable factors make it advisable to resort to a brake rather than relying on the self-locking characteristics of the worm gearing. ■

Originally Posted by Lyle Brown

in peoples experience is this behaviour observed from "typical" industrial reducers?

In my case a resounding YES ■

Originally Posted by designer

Ask your worm gearbox supplier if his proposed gearbox can be back driven.

That's the best way

Ganesh,

You could find out the worm ratio in a multi stage reducer from the catalogue after a lot of pain and frustration. (Why are the catalogues so complex?) Then you have to decide if the referred inertias are upstream or downstream of your application. Someday soon we are going to be offered recirculating ball worm reducers (maybe they're already here) which will shift the friction goalposts.

It is safer to consult the manufacturer after you have worked out the reverse torque. You could be a pioneer...I've yet to come across a conveyor drive spec which made adequate provision for the situation. It's not the same as a reversible conveyor and previously we've just fitted a gradual release holdback whenever necessary. Good thread.

Personal experience:: Last Tuesday night I was parked up, waiting to go onto the bay at Tesco in Blackburn, UK. I observed the steering wheel of my trusty Scania turning slowly as the caster angle settled the vehicle to the hillside slope. It was -3 degrees so the coefficient of friction between ice and rubbber was in evidence. Yes, it's pretty typical. ■

Things I learnt early on in my engineering life about worm gear reducers -

1) they can be back driven

2) they have a very poor starting efficiency

3) it's surprising how many gearbox salesmen don't seem to know about (2) ■

sganesh

Here's my original response

Originally Posted by designer

Ask your worm gearbox supplier if his proposed gearbox can be back driven.

That's the best way

If you haven't yet followed this advice, then in the words of Nike, "JUST DO IT" ■