Welcome! Log In Create A New Profile Recent Messages


need some advice on gears

Posted by jjtjr 
need some advice on gears
September 19, 2020 05:50PM
My rear end gears failed on my likamobile, trying to figure why they failed. here is photos of them. all the pinion and axles gears are damaged. I can see a tooth broke off and damaged the other gears. the question is why did it break a tooth. Was the gears a poor design to start with or was it metal failure. I will be cutting new ones but need to know why first so it doesn't happen again. Don't know the HRC number or type of alloy used.
From what I been reading on spur gears you should use s45c steel, pressure angle of 20 degs and a HRC # 50 to 60. any help would be good.

Re: need some advice on gears
September 20, 2020 08:34PM
What is the wear mark on the center of the teeth from? My only real experience with spur gears is in clocks. There, the only issues tend to be shock loads and debris. Splitting the gear in half is an awfully dramatic failure.
Re: need some advice on gears
September 21, 2020 07:56AM
Tooth failure on a gear is not un heard of but is rare. Most often it’s caused by something ales getting in there. Those gears most likely are off the shelf items in England. Any good gear and bearing shop hear may be able to get then in less then a week.

On my EX the eccentric strap bearing cap came loose and the balls flew out, one landing in the gears between the engine and differential gear. The differential gear being bronze allowed the steal engine gear to drive the hard bearing ball into the bronze gear.
I took a Dremel slitting wheel to cut the ball in half to get it out. The ball did dimple every tooth on the engine gear but the Dremal grinder cleaned up every tooth nicely.


Re: need some advice on gears
September 21, 2020 03:22PM
HPC Gears in Chesterfield UK are the most likely source and are a very good company to deal with. Look at their catalogue.


Re: need some advice on gears
September 24, 2020 05:31PM
just did a rockwell test on the gears the small ones are 40 HRC and the large one is 48 HRC. From what I've been reading pinion gears should be between 50 to 60 HRC maybe that's why they failed
Re: need some advice on gears
September 25, 2020 04:00AM
I wonder if the recommended hardness should be the hardness of the surface after induction hardening rather than hardness through the gear? Are they in fact too hard through and therefore brittle? You really need a metallurgist to advise on this.

Re: need some advice on gears
September 25, 2020 06:41AM
I would say your failure is Foreign Object Damage (FOD). We use this terminology a lot in the aircraft industry. The first picture you posted is pretty good evidence that this is what happened. Then Rolly confirmed that it is possible. His gears are soft and can withstand the FOD. Also, this design is self lubricating to some degree.

Your hardness is fine. The teeth are hardened for wear and longevity. These gears are ground to a low RMS to prevent friction. These gears will last a long time. However, they aren't resilient to particles that get in there. Your system is an open one to the environment.

You can switch the design to more of a Rolly configuration or replace this one. I know that you can get a new set at Boston Gear. Give them a call to discuss your situation.

Boston Gear Website
Re: need some advice on gears
September 26, 2020 06:53AM
Not sure how any foreign object got into the sealed diff case.It was cleaned and assy. then gear grease was pumped into it with a grease gun It's been in operation for some years. When I opened it up the only thing I found were broken teeth no other objects.From what info I've found on pinion gears they should be case hardened to 50 to 60 HRC. At this time a new set was ordered and I also ordered 4140 bar stock to make new gears and have them case hardened. to 50- 60 HRC.
Re: need some advice on gears
September 28, 2020 06:26AM
I didn't realize that the Likamobile had a sealed rear end. Then I would say it's a definite gear failure. It is either poor material or poor hardening. By the pictures and this is just from seeing a few material failures over my Engineering career, the appearance is of a brittle, face centered cubic structure like a martensite.

Anyway, you're on the track to recovery. Your new gears seem to me to be more robust.

Good luck with putting her back on the road! Keep me posted for I'm eventually building a rear end for my steam car. My rear end will be a dry one as opposed to a wet engine/rear end. I'll be spraying oil on it before every run much like the typical 1908 Stanley versions.
Re: need some advice on gears
September 28, 2020 07:18AM
Yep the rear is sealed, What I found is the little gears are cast and the large gears are steel, what type I don't know. The ones that I will be making will be only case hardened. In the meantime I will be installing the ones that I'm getting to get the car back on the road.I will be performing a HRC test on the gears that are coming soon. I think they will be the same HRC as the ones that failed I will post the results.
Re: need some advice on gears
September 28, 2020 03:49PM
Cast steel sounds wrong for gears - my guess is that is very likely the problem.

Re: need some advice on gears
September 28, 2020 07:21PM
Does anyone know how many thous depth of case hardening should be for spur gears just the teeth
Re: need some advice on gears
October 08, 2020 11:14AM
Just for the heck of it, insert two straight shafts into the gearbox and measure their parallelism quite carefully. If they are out a little bit, the forces can be concentrating on one side of the gear teeth. I ran into a gear box some years ago, built by a big name gear company, and someone slipped up on quality control --- with similar results to yours.
Re: need some advice on gears
October 09, 2020 07:53AM
Google " heat treat forum spur gears " for discussions about this subject.
Re: need some advice on gears
October 10, 2020 05:31AM
I searched on google for that forum, but did not get any results, do you have a link Thanks
Re: need some advice on gears
October 10, 2020 11:04AM
General google search +heat+treat+forum+spur+gears

Materials, gear_material_steels_for_industrial_gearing_13180

Specific thread, heat-treating-gear-out-4140-steel

Search of practical machinist
Re: need some advice on gears
November 23, 2020 11:37AM
Rolly, I need you help on this one, I received new gear. Installed them moved the car 20 feet and it wouldn't move after that.When I installed the new gears there was no binding. They rotated freely. I opened the diff up and this is what I found. the pinion gears had broken teeth. the axle gears were ok. the new gears were cast. I don't see any interference with the axle gears. Is the problem the cast gears being to weak. The other half of the diff looks the same. Could the weight of the car be the problem its 1500 pounds dry.

Re: need some advice on gears
November 23, 2020 12:23PM
Is the problem the cast gears being to weak?
Sure looks like it.
Why are they off center? Is that the way it’s designed?
Looks like you need harden steel gears.
Re: need some advice on gears
November 23, 2020 06:29PM
The off set is the way it was designed. If you will be around this week I have them here in florida and can stop over and you can look at them real time and tell me what you think. Don't know why cast gears were used. It looks like the weak link.
Re: need some advice on gears
November 23, 2020 06:37PM
The plan is to machine new gears using 4140 bar stock and then have the teeth case hardened to between 50 to 60 HRC around .020" deep. The original gears are cast but my car is heavier because its a 4 seater and that may be the problem.

Edited 1 time(s). Last edit at 11/23/2020 06:39PM by jjtjr.
Re: need some advice on gears
November 24, 2020 03:21AM
I’m around Jim
Looking at them won’t solve the problem.
Re: need some advice on gears
November 25, 2020 06:26AM
I don't think the involute is correct. The pressure angle appears too low. What was ordered?

I'm going to order the attached gear for outside my differential. It is a 14-1/2 Deg angle. Could you need a 20 degree angle to match your set up? I know that old gears were typically 14-1/2 Deg. That's why I need to go with this gear. Just a thought, I would think that the spider gears would be 20 degrees.

If anyone can help you figure that out, it would be Rolly. Note that the higher the angle, the stronger the tooth. The lower angle puts less load on the bearing.

Sorry I didn't think about this sooner.

Hope this helps,

Re: need some advice on gears
November 26, 2020 05:16PM
Looks like one of the gears were installed backwards. Can't see the factory designing it the way it looks. Maybe someone that had the car before you installed the gears wrong.
Re: need some advice on gears
December 13, 2020 10:05AM
About to start cutting new gears from bar stock, need some advise. Im going to ruff cut stock Is it best to first cut OD to size then bore centre hole. Or ruff cut Stock then bore centre hole, then turn again on centre hole and do finale cut of OD. i will be using a involute cutter with rotary table on bridgeport.
Re: need some advice on gears
December 13, 2020 10:12AM
I can't talk about gears, but as a general rule, when machining anything, I always like to leave a little stock on both the faces and the OD for final cleanup. For example, I just built a fixture to install helical gears onto a crankshaft -- and the thing warped on me while milling in some of the features. It was easily corrected because I left stock for a cleanup surface grind.
Re: need some advice on gears
December 27, 2020 06:08AM
Started to make pinion gears from 4140 stock. Once made they will be case hardened about .015" deep to 50 - 60 HRC

Final step will follow

Edited 1 time(s). Last edit at 12/27/2020 06:10AM by jjtjr.
Re: need some advice on gears
December 30, 2020 06:34AM
Finished one of four, second one almost done. Not having a CNC mill makes it very slow, I have an old j head with DRO and motor drive on x axis. Making .030" cuts to a total of .212" thats about 5 passes per tooth but the feed rate is 8 mins per pass. Have to cut 16 teeth. Don't want to dull the cutter. Using lub on cutter.
The wheel gears will be next only have to make two but 32 teeth.
Re: need some advice on gears
January 03, 2021 02:36PM
All four gears are now done.. Heat treating tomorrow. It took 1 day per gear, not having a CNC mill makes it go slow.

Re: need some advice on gears
January 03, 2021 04:21PM
Looks really good Jim!
Re: need some advice on gears
January 08, 2021 02:09PM
And i' m not done, yet since I have my machines already setup for gear cutting, just purchased bar stock of 8620 alloy to make another set of gears. This time I will machine it a little different instead of 4 blanks it will be 2 double the length then cut the teeth after that part and face, then to heat treatment for .030 case depth and harden to 50 to 60 HRC.

Sorry, only registered users may post in this forum.

Click here to login

All files from this thread

File Name File Size   Posted by Date  
IMG_0160a.jpg 948.7 KB open | download jjtjr 09/19/2020 Read message
PA080001.JPG 852.4 KB open | download Rolly 09/21/2020 Read message
20201121_163142 c.jpg 971.6 KB open | download jjtjr 11/23/2020 Read message
Gear selection.jpg 908.1 KB open | download Rick.H 11/25/2020 Read message
423E79D4-2DD0-4D8B-8CA1-55950E518AD5.jpeg 392.4 KB open | download jjtjr 01/03/2021 Read message
24BD191D-10E5-4AA9-9960-1F2BCCC0F0B4.jpeg 390.5 KB open | download jjtjr 01/08/2021 Read message