Aluminum gear?

It looks like I've solved my shaft breakage problem with my model boat gearbox, thanks to help from people here. My problem now is that I'm stripping the machined nylon gears when I really push the unit hard, and especially when it's cold out (teeth getting brittle?)

Other than a molded nylon gear, the machined MC901 that I'm using seems to have the best specs of any of the plastics so it looks like some sort of metal is in order.

The pinion on the motor is brass so I was thinking of making the spur from 7075 aluminum and having it hard coated (hard annodize) for wearability. Here's a shot:

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The motor turns 30,000 rpm plus and the ratio is 1.5:1.

Would this work? Any other suggestions?

Reply to
Terry Keeley
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Reply to
William Noble

Nylon does that. How cold is cold, in degrees?

There are other plastics used for gears. Delrin, for instance. Or the traditional choice, machined cloth-reinforced phenolic.

What's the torque? The wattage or horsepower?

If the gear will be metal (brass or aluminium), the pinion would need to be steel. Aluminum on brass won't last long.

Joe Gwinn

Reply to
Joseph Gwinn

A few different failure modes come to mind.

First mode is if the shafts are not held in place properly (bearings moving or shaft flex) This is componded by high loads, strange vibration modes, etc. But if the teeth are allowed to pull away from each other, the contact point goes higher on the tooth and promptly strips the teeth.

Second mode is where the teeth wear out. You might want to run the unit for a while, take out the gears, inpsect carefully for signs of wear. Youshold be able to use a 10x magnifing glass to spot teeth that have wear on one side so the tooth is not symetirical.

Since you find low temps a problem, I'd suggest that the nylon is getting brittle at the lower temps, does not have enough impact resistance to withstand the forces you are putting on it.

If you post the exact specs on the gears (diameter, pitch, tooth count) as well as the torque (or HP) and rpm, someone can show you how to do a tooth loading calculation.

I suspect that you will eventually have to go to a steel drive gear and a brass driven gear. Your tooth load> It looks like I've solved my shaft breakage problem with my model boat

Reply to
RoyJ

You basically are overloading the gears. Do them with filled Delrin and do larger surface gears, i.e. wider ones. Plastics tend to not wear until a certain point and then they wear rapidly. One thing not to do is use anodize on aluminum. The anodize coat is a nice fine abrasive and will quidkly shred anything else. The gear surfaces need to actually be polished for better transmission and cut nylon or other gears will have rough surfaces that will quickly degrade. Better to have a cast gear for something like this if you insist in using plastic. Also make sure that the tooth count is even on one gear and odd on the other as this will tend to decrease the wear as another poster noted. You may want to go to steel gears (in an oil bath) for higher power transmission and speeds.

-- Yeppie, Bush is such an idiot that He usually outwits everybody else. How dumb!

Reply to
Bob May

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How are you machining the gears? What pressure angle/profile are you using? How are you verifying the tooth profiles/dimensions before installation?

No silver bullets, but....

Put an extra tooth on either the drive or driven gear to get a hunting action to avoid always having the same teeth come into contact. Buzz-word is "relatively prime" meaning that the tooth count for the two gears don't have a common factor. One tooth more-or-less should not change the c/c distance of the gears a great deal so you should still be in adjustment range.

Given the rpm and tooth count you could be sttting up some sort of torsional viberation far above the range of human hearing.

If you are using straight [across] cut gears, helical gears can help as there is a more gradual engagement/disengagement.

Use two gears on each shaft. If you are using helical cut gears, try to get a left/right pair [called herringbone] so the opposing helix angles will generate opposite sense end thrust, sparing the bearings some axial thrust load.

If you are having the gears custom cut, try using one of the higher performance [and higher cost] engineering polymers such as minlon [mineral filled nylon], polysolfone, etc.. Fiberglass, (and most likely carbon fiber reinforced versions {I can hear the cash register ringing} by this time) filled/reinforced versions of these are also available, although these can be abrasive. see:

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There are also the more traditional thermo set laminates with woven reinforcement such as micarta and formica specifically developed for gear use. Another version of these is used for electrical panels, so be sure to get one designed for gear use.

see

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Given the size, power and speeds you are are running, you may have a situation like the nitro fueled drag sters where they make one full throttle run and rebuild the engine/clutch/transmission in addition to repacking the parachute.

Its fun to push the envelope, but it can get expensive too.

Good luck and let us know how things turn out.

Unka' George (George McDuffee) .............................. Only in Britain could it be thought a defect to be "too clever by half." The probability is that too many people are too stupid by three-quarters.

John Major (b. 1943), British Conservative politician, prime minister. Quoted in: Observer (London, 7 July 1991).

Reply to
F. George McDuffee

Last time I was out it was about 6C (42F).

Was told on another forum to try PEEK, looked at the specs and carbon filled has a tensile strength up to 40K psi. Was able to track down a min. order of 1 foot of 1.25" rod, a little pricy at $450!!!

Not sure the exact torque, might be around 3 HP.

Thanks, that's what I was wondering. How about stainless steel and brass?

Reply to
Terry Keeley

Thanks, that's the sort of info I'm looking for. I found that some are using hardened steels gears, might have to go that route...

Reply to
Terry Keeley

The plastic spur gear is a stock item, the brass pinion was machined with the proper cutter, although not a high dollar one. The spur has 30 teeth and the pinion 20, profile is metric MOD1. How does one "verify"?

Changing the number of teeth or arrangement would mean a complete re-build of the unit, so I'll avoid that at all cost.

Looking into carbon-filled PEEK, but unles I can get a "sample" somewhere it's cost prohibitive.

Funny you mention that, I'm a huge drag racing fan and have been to several NHRA events. Problem is though that I'm not even getting to half track before smoking the tires :)

..............................

Reply to
Terry Keeley

Not all that cold. The gears were overloaded, it seems.

Ouch. Brass is looking better and better.

Three HP. That's a bit too much for plastic, especially with such a small drive pinion: 1.25" in diameter with 20 teeth driving 30 teeth, at

30,000 rpm.

Stainless steel on brass works fine. But don't forget the oil film. At

30,000 rpm, how to get the right amount of oil onto the meshing surfaces will require some thought. Oil baths don't work at this speed. Nor does grease. The oil or grease just get pushed aside. Mist oiling could work.

I would also suggest changing the tooth counts slightly, as has been suggested, to a mutually prime set of counts, to spread the wear out. Like 29:19= 1.526316.... How accurate must the 1:5 ratio be?

Joe Gwinn

Reply to
Joseph Gwinn

The speeds and loads you are running are at the very edge of the envelope, real aerospace stuff.

Exact tooth profile and size is not too critical for low speed/low torque/power operation and many older slow speed machines such as steam engines, lathes, washing machines, apple pealers, etc. could and did get by with cast iron gears with the teeth as cast.

The grade or precision of gears is indicated by the AGMA classification. 3 through 16 with the accuracy and price going up with the number. see

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Verification at the level you require needs special equipment. The purchased plastic gear will most likely have an AGMA grade specification. Even with the best cutters, it is doubtful your pinion gear has as high AGMA class profile as your molded gear.

This is the reason that the hyper-precision aero-space and fire-control units are so expensive, as the gears with the required accuracy approacy what they would cost in solid gold.

My first though would be to use 2 plastic gears. The improvement in geometery may offset the like material operation.

See if your supplier offers shaved plastic gears in the right size. As molded are good for less demanding applications.

Can you put two gears of the same type on each shaft or use wider gears?

Given your speed/power requirements & size of gears, set-up is critical. I suggest you use prussian blue, candle smoke or what ever and verify the mesh pattern of your gear teeth as installed. It can be a royal PITA to get everything aligned, but it is critical.

Given the AGMA quality levels you require, most likely a one off shop hobbed 1.0 mod gear is not going to cut it. Dig deep in your posket and get the AGMA quality gears you need and double check the setup.

Shows you are not overbuilding. Ideal is where the engine/gearbox explodes as you cross the finish line proving there is no extra material/weight.

Unka' George (George McDuffee) .............................. Only in Britain could it be thought a defect to be "too clever by half." The probability is that too many people are too stupid by three-quarters.

John Major (b. 1943), British Conservative politician, prime minister. Quoted in: Observer (London, 7 July 1991).

Reply to
F. George McDuffee

Thanks for the good info, I'll have a look at changing the tooth count but it might mean having to change the center to center distance of the shafts. I wasn't really having a wear problem I don't think, more just overloading by the way the teeth were stripped off quickly.

I've ordered a stainless gear from

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and will give that a try...

Reply to
Terry Keeley

Brass (what most call brass) isn't very good at gliding. If you replace brass with bronze and do that on hardened steel, it's good. The steel needs a HRC of 52...54.

Nick

Reply to
Nick Müller

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