Improve Longevity of Worm Gears?

[Chris Kelley]

I'm designing something that includes a primary (worm) shaft rotating at approx. 1,800 RPM that must drive a perpendicular shaft with about a 40:1 ratio. The gears will be inclosed and lubricated, and the secondary (worm wheel) shaft will be under less than 5 ft. lbs of torque. For the first prototype, I used a 1/2" diameter stainless steel worm and a roughly 1.4" diameter bronze worm wheel with a 40:1 ratio, lubricated with graphite grease, and the gears fit together well, but wear on the bronze gear caused a failure after around 20 hours of running. We want this to withstand at least 150 hours or continuous use, so are there any ways to greatly increase the lifespan of this concept without entirely throwing away the use of a worm gear system? Are there any materials that would last longer than stainless on bronze in this high RPM application? I'm open to redesigning the gears themselves if there's a way we could hit our longevity target without using a spur gear larger than 2.5" diameter. I'm hoping to avoid adding more gears to the equation for stepping things down.
Any insight on this would be greatly appreciated!

 

[Chris Kelley]

The prototype gears:

 

[sfriedberg]

That doesn't seem like an outrageously high RPM for a worm gear reducer. Not an expert in these things, but there's probably a reason many such reducers are lubricated with oil with extreme pressure additives and safe for red metals (brass, bronze, etc). Link Another Link
In addition to changing your lubrication, you might doublecheck the worm/wormgear clearance (there should be some!) and measure the worm temperature in operation.

 

[Milland]

Ditch the stainless worm and go with ground hardened steel, and ditto don't use a graphite-added lube - just a proper oil safe for red metals, perhaps synthetic to get best life. Get recommendations from the gear manufacturer.

Make sure you don't get moisture buildup inside the gearcase, there's vents available that will allow water vapor to purge while retaining oil.

 

[Milland]

I checked the picture a little more closely - looks like you're making your own gears? Unless you guys know your gear chops, you're better off getting them made by companies in the business. Poor tooth form and accuracy will doom a gearset.

 

[john.k]

A worm wheel must be cut on a proper worm gear hobbing machine..........and the gear mesh must be set correctly......if form or setup is wrong,all lube is wiped from the faces on the 'entering side' and the gears run dry..........mesh should 'draw in' the gear lube ......which must never be an EP oil.....EP oils kill worm drives.

 

[Garwood]

Graphite is kind of a lubricant, but also an abrasive. Do not use graphite on gears.

Why can't you buy a premade gearbox to do this?

 

[john.k]

The worm wheel in the pic does seem to have proper form.....its not a spur gear ......and so Id suggest correctly setting the mesh......first contact on the 'entering' side will be at least 1/3 way across the face of the gear .....this pulls lube into the loaded contact area.........incidentally,worms are often not hard ....for instance,Ive recut rusty Garwood winch worms with HSS tooling,and they were not hard......the worms in truck diffs are not very hard either.,and they last forever ,if correct lube is used.

 

[Laverda]

I have a 1940's worm gear drive on a vintage vehicle. It recommends 140 weight gear oil only. Any other lubricant the manual says will cause eventual failure.

 

[Phil in Montana]

A worm gear is the highest friction gearing you can have, I would sub the machining out, or buy all the right machinery to build the gear sets. Use 90wt or better gear oil for brass. Make the worm wheel from sil bronze and the worm from heat treated 8620...

 

[john.k]

I use steam cylinder oil in worm truck diffs ,its called Valvata 100 and is very thick and sticky.........the original oil in my Coles crane worm diffs was near black,but had a red tinge when viewed against a strong light.....Ive seen similar oil in WW2 truck diffs ,I thought it was DTE extra heavy.....but its considerably thicker ........strong smell ,but not EP smell ......never use EP in a worm drive.

 

[ratbldr427]

We used steam cylinder oil that was 600 wt. until every thing now uses synthetic oil like Mobile SCH 680. Have some hand drive small worm gear boxes that are greased but don't believe I have ever seen a motor drive that used grease. Doesn't matter how trick it is grease is just not the right lube. It gets wiped off the tooth faces and dose not recirculate like oil and besides does a piss poor job dissapating heat.

 

[guythatbrews]

If the overhang of both shafts is the same in operation as it is in the pic it's way to much. Deflection is likely changing the tooth contact.

If power tools are any indication a hardened worm and the proper grease should last a long time, if oil lubrication doesn't suit your design. I think aluminum bronze worm gears are pretty common and that stuff is tough.

 

[CarbideBob]

Grease is not a very good lube for worm gears.
To much sliding friction and not enough oil. Worse yet if only driven one direction as it balls up.
You should find that a good gear lube will give you 10x the life or more.
Life here is a big worry and the numbers given in the not understanding what the heck is going on.
20 hours...... Something is so very big wrong other than just lube.
Bob

 

[john.k]

Kirkstall truck worm diffs were made of a bronze called 'Colphos'.......which was claimed to be the best alloy available..(in 1950)

 

[EmGo]

Jesus H Christ ....

Yes, you can run a helical against a worm. That gives you point contact. Single enveloping is the next step up, that gives you line contact. Double-enveloping is a step above that, gives you area contact. Each step will decrease your surface loads one step, therefore help longevity. Single enveloping and above really needs to be done on a hobber, I've seen people make them on old horizontals and all that but for producing more than one, up to you but pulling hair out is not my fave thing.

After that, there's materials and lubrication. But I usually go for basics first. Bronze is common in wormgears so you're not way off on that, can play with that a little too but in your case, probably will not overcome the point contact high surface loads problem all by itself. The helical-against-worm thing is cheap but not very robust.

If you want to insist on sticking with that, try carburized steel against carburized steel. That works too. Or nitrided, or whatever ... many ways to get hard, as she said

 

[CarbideBob]

Yes, you can run a helical against a worm. That gives you point contact. Single enveloping is the next step up, that gives you line contact. Double-enveloping is a step above that, gives you area contact.

One has to assume the OP knows about contact in worm gear work and forms and the loading as the screw turns.
Darn it seems easy.. and it seems complicated. I am sliding over and up or down a bit at the same time.
My load is contrast but my load per unit area is often not .
Gears that are not straight 90's are a deep well of problems.

 

[EmGo]

One has to assume

I'm learning that one better not assume anything around here

 

[johansen]

Automotive windshield wiper. Wont be exactly 40:1 but 5 foot pounds is nothing for them.

 

[john.k]

I suspect the problem may be in a lack of accuracy in manufacture of the worm...pitch/,tooth form/surface finish/,runout,/