Looking for highly probable non-backdriving worm gear reducer. 4

[windell747]

Aloha,

I've been working with Boston Gear to find a right angle shaft non-backdriveable worm gear reducer. The best choice they have to offer is a two stage worm gear reducer that weighs 28lbs. The first stage has a worm lead angle of 8.47deg and the second stage has a lead angle of 14.23deg.

According to my worm gearing documentation, the non-backdriving condition is satisfied when...

worm lead angle < tan^-1(coeff. of friction).

The coefficient of friction I used is 0.145 for a hardened steel worm/phos. Bronze gear combination that I obtained from a reference table. This gives me a friction angle of 8.25deg. As you can see 8.47deg>8.25deg. The non-backdriving condition above is not satisfied. I used the static coefficient of friction since the input shaft will not be moving when the motor is shut off (using a stepper motor).

The motor must be shut off for my application and I’m not allowed to use any electrical brakes or clutches. So the gearbox will need to be entirely non-backdriveable.

I’m looking for a worm gear reducer that meets the above non-backdriving condition. Meaning one that has at least one of its worm lead angles <8.25deg. It is completely fine for the reducer to have several stages, however one of the stages needs to have a worm lead angle <8.25deg.

I don’t need guarantees of non-backdriveability (no company will guarantee this for liability reasons), I just need high probability. Below are the some requirements I’ve come up with for the reducer.

Couple things to point out…
-gearbox will be INDOORS and will NOT be exposed to any vibrations, high loads, shocks, rainy weather, or temperature.
-will NOT be used on a safety system.
-usage will be <10min/hr

Question: Would you guys be able to point me to a company that has worm gear reducers that have a higher probability of non-brackdriveability? Or in other words is known to manufacture highly non-backdriveable worm gear reducers? If so, can you please direct me to a website of theirs? As many responses as possible from highest probability to lowest will help!

Mahalo Nui Loa,
Windell Jones
University of Hawaii High Energy Physics Department.


WORM GEAR REDUCER REQUIREMENTS:

-300:1 to 400:1 reduction. (Triple reduction is fine)
-Non-backdriveable (self-locking) (worm gear < 50% efficient using static coefficient of
friction).
-WILL NOT USED ON A SAFETY SYSTEM.

OUTPUT TORQUE CAPACITY: approx. 400 in-lbs (can be a little less)
SHAFT CONFIGURATION: right angle shafts
WEIGHT: light as possible.
APPROX. INPUT SHAFT SPEED: 120RPM
OUTPUT SHAFT SPEED:
-0.4RPM (w/300:1 ratio)
-0.3RPM (w/400:1 ratio)
GEARBOX USAGE: less than 10min/hr.
ENVIRONMENTAL CONDITIONS: The gearbox will be indoors and will NOT be exposed to rainy weather, loads, shocks, vibrations or temperatures.

 

[Philrock]

I don't know if you will be able to find a catalog reducer that will meet your requirements, however, Martin Sprocket & Gear lists worms and gears that could be used to build a unit that meets your requirements. Unit would be 2 stages and one stage would have a helix angle of 4 deg 34 min.

http://www.baymachinedesign.com

 

[willdo]

Could you use a one way or clutch bearing on the output of your reducer to keep it from backdriving? Emerson-EPT may be a good company to talk to on this since they offer both speed reducers and a variety of non-electrical clutches.

 

[tbuelna]

windell747,

"The coefficient of friction I used is 0.145......"

Your suggested coeff. of friction value is way too high. For the worm gear set to operate properly, it will need to be lubricated. Even a hardened steel worm on a phos. bronze gear will gall if not lubed. I would recommend a mu (coeff. of friction) of .05 or less for your analysis, for an interface with grease lubrication.

Good luck.

 

[sacem1]

Windell747:

You should be able to solve your requierements with a DODGE COMBINATION TIGEAR SIZE C150 speed reducer it is composed of a single entry worm (60:1 ratio) and a helical secondary reduction of 5:1 which gives you a 300:1 overall ratio.

The reducer has the following data:

Output torque 1658 lb-in @ 870 rpm input (it increases when you lower the input speed so you should check OK at your 120 RPM input speed.

It has an overall efficiency of 76%

Supports an OHL (over Hang Load) of +1,000 lbs

Weight is around 72 lbs and should cost around $1,000

The 60:1 single entry worm is self locking, period.

you can visit them at:

http://www.industry.net/dodge.rockwell.automation

or (864) 297 4800

Hope you get what you need.

SACEM1

 

[windell747]

Thank you very much for your posts! Your responses are showing me the difficulty associated with meeting my spec. tbuelna, I understand your concern for my high coefficient of friction. The reason I use the static coefficient is because the stepper motor isn't turned off until the worm and worm gears are at a full stop. sacem1, thank you for your suggestion, but that gearbox is way to heavy duty for me. I'm also not allowed to use a mechanical brake or clutch. One option I could go with is build the gearbox. I know building my own gear reducer with purchased gears would meet my spec.

 

[sacem1]

With the suggested reducer you DO NOT NEED a brake the reducer is selfbraking (a 60:1 single entry worm and gear are always self braking except if you build it with a huge gear and a tiny diameter worm, but that is not the case, usually)

With the weight I can not do much for you, the idea could be to use an Aluminium case (instead of the heavy cast iron one), that should cut the weight by one half.

There are several aluminium case single stage worm reducers in the Grainger catalogue and you could team it with a second parallel shaft one but that will make you do all the couplings of the output/input shafts but I guess it will be easier than making all the work by yourself.

If you do not find another supplier we could manufacture one with those characteristics specially for you.

Good luck

SACEM1

 

[windell747]

Aloha SACEM1,

You said that that your company can manufacture a gearreducer that would meet my specs. Could you tell me more? Maybe a price range?

Mahalow,
windell747

 

[sacem1]

Hello windell747:

That was not intended as a sales pitch but as a way to solve your problem, anyhow if you want please contact us directly at:

sacem@viabcp.com

and I will be able to send you more specific details

Cheers

SACEM1