Bevel Gear Set-up 2

[artbuc]

Power is transmitted to my Ariens mower through a gearbox mounted on the mower deck. Ariens has been using this design for at least 25 years and before that it was used by New Holland. The mower is no longer in production but Ariens will build one for you from parts which is what I just did. Unfortunately, the new gearbox had excessive endplay on the output shaft (0.028" vs spec of 0.003/0.008")which could not be fixed. Usually there are shims which can be removed to reduce endplay but this unit had excessive endplay with no shims. The replacement gearbox was the same. My dealer did a non-Ariens approved modification to reduce endplay (something he has done many times over the years) but now the box has a high pitch whine at full speed and an intermittent scraping noise at half-speed.

Ariens Engineering Department has set-up another gearbox and is sending it to the dealer. They are not forthcoming about what caused the original problem other than saying it was due to an accumulation of tolerances. This doesn't strike me as plausible.

The box design is somewhat crude. It is aluminum diecast with the bearing races for the vertical output shaft located by non-machined shoulders in the diecast. The horizontal input shaft bearing races on either side of the input gear are adjustable to set input shaft endplay and gear backlash. The gears are straight bevel and have no markings to set mounting distance or to identify mating teeth. The service manual does not specify gear mounting distance but says backlash should be 0.005/0.015".

Unfortunately, I think all of the new Holland/Ariens engineers who know about this box have retired and Ariens says they will obsolete the part instead of investing much effort to find the problem. I am expecting the third box will not be much better than the first two.

Can someone give me some suggestions on how to set-up this box? Or, where can I find a person or shop that can set this box up for me? Thanks for your help.

PS I will try to post a drawing of the gearbox although I think you all can probably visualize it.

 

[artbuc]

I have attached a drawing of the Ariens gearbox.

 

[MikeHalloran]

Wow, talk about product/brand loyalty.

I'm guessing your dealer is breaking in a new tech on the old stuff, and he set yours up just a little tight. Give him a chance to rectify that.

If the gears' axial position were settable with screws, e.g. with threaded collars around the bearings, I'd start with the gears lightly pressed into each other (zero lash), and back off the adjusters equally until there was maximum backlash, then run in the complementary adjusters until there was minimum backlash with the shafts pressed toward each other. Now, all you have to do is figure out how to do something equivalent with shims.



Mike Halloran
Pembroke Pines, FL, USA

 

[artbuc]

Thanks Mike. Were you able to see the drawing in my second post? The input gear axial position is adjustable with a threaded collar on the input side and shims on the case cover on the opposite side. You can see the two pins on the input shaft which hold the bearings against their races. When you screw in the adjustment plug on the input side, it moves the input gear away from the output gear to set backlash at 0.005-0.015". Once you get that, you measure the gap between the case and case side cover so you can add the right shims to hold backlash. Then you backoff the threaded collar adjustment plug and tap the input shaft back to establish input shaft endplay. No problem with any of this. The hard part is establishing the mounting distance of the output gear. Ariens provides shims to install between the output gear and the bearing immediately below it. However, there isn't any reference in the service manual telling you how to establish the mounting distance.

The person who modified the gearbox has been working on these boxes for 25 years and I have known him almost that long. He added custom shims between the case shoulder and lower bearing race on the output shaft. This allowed him to then come back in with stock shims between the bearing and output shaft shoulder as shown on the drawing. This modification should not affect gear performance. The box is set-up loose as a goose with shaft endplays at 0.008" and backlash at 0.015". The original box had 0.030" backlash and 0.028" output shaft endplay. The box ran quietly with this excessive play but I don't think it would have lasted very long.

Having said all of that, how do you bring the gears together if you don't know the mounting distances? Do I just bring the back angles flush? Also, what do you mean by complementary adjusters? Thanks!

 

[MikeHalloran]

I had not seen the drawing when I commented, so I was working with an imaginary gearbox, having threaded adjusters on all the bearings.

I think basically you have to start with zero lash to get the gears in the proper relative position, then move them away from zero lash to the minimum backlash setting, set the shims or collars or whatever so the gears can't move closer than that, then use the remaining shims collars whatever to not allow them to exceed the maximum backlash.

Of course, that nice idealized sequence doesn't fit nicely with the particular gearbox design, which makes my head hurt.



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Upon further inspection, it appears that the output gear is fixed in place by the upper output bearing bore in the housing. Shimming the cup of that bearing could tighten the gear mesh, but no shim is specified.

Then the output shaft endplay is set by the shims at its lower end.

Then the input gear is positioned, by the side cover gasket thickness, then the input shaft endplay is set with the adjustable plug on the input shaft.

That's just me looking at the assembly drawing. The approved procedure may be considerably different. As you said, the factory engineers may not know the _real_ factory procedure, that probably retired or died with someone.





Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

artbuc,

There are two adjustments you must make for each shaft. One for the correct preload on the tapered roller bearing pairs, and one for the correct mounting distance of the gears.

Looking at your picture, your adjustment procedure should start by setting the correct bearing clearance on the vertical shaft by selecting the correct washer thickness (ie. .005, .015, .025 or .032 inch). There are no provisions for setting the gear mounting distance on this shaft. Next you should set the correct backlash on the horizontal gear shaft by selectively fitting the correct thickness combination of gaskets on the right side, and then setting the correct bearing clearance by adjusting the plug on the left side.

As for the recommended RT shaft end play of .003 to .008 inches, that seems a bit excessive for the tapered bearing arrangement shown. A backlash of .005 to .008 inch is probably OK for this class of gear, but you should also do a contact pattern check to make sure the gear mesh contact is fairly centered on the tooth, heel-to-toe.

After making your adjustments, make sure all your fasteners are properly torqued and locked.

Hope that helps.
Terry

 

[artbuc]

Mike, your deduced set-up procedure is 100% correct as stated in the service manual. One interesting note: the drawing/set-up procedure in the service manual makes no reference to shims on either side of the top bearing on the output shaft. So, as you stated, the location (mounting distance) of the output gear is fixed by the top shoulder in the diecast case. However, the parts diagram does show shim sets on either side of the top output shaft bearing. That is the basis for my question. I can change the top gear mounting distance by adding shims but I don't know what the mounting distance is. I read one article which said that the only say to establish the correct mounting distance in my situation is trial & error using a gear compound to achieve a good tooth contact pattern. I would have no idea how to do this correctly and I don't know how to find someone who does. I could start by setting the back angles flush and moving each gear back the same amount to get the right backlash. Of course, the output gear may already be too high and adding shims will just make it worse.

Terry, you are also 100% correct in saying that the output shaft endplay is established first. The specified backlash is 0.005-0.015". The first two gearboxes came from the factory close to 0.030". From what I have read, determining the proper contact pattern is more of an art than science. Do you think someone like myself can do it properly?

Mike, why does this box design make your head hurt? The main problem I see is that the diecast case is not machined to make sure the shafts are perpendicular. The only machining is cutting threads for the adjustment plug.

PS If I would have know how much fun gear design is, I would have been a mechanical engineer!

 

[artbuc]

Would this method work on straight bevel gears?

http://www.norcalttora.com/~dick/Dana Axle Stuff/Dana ring and pinion pattern.pdf

 

[MikeHalloran]

Here's what makes my head hurt about that gearbox:
- It requires adjustments during assembly.
- The adjustments are not obvious, and interact with each other, and are laborious to effect.
- It appears that use of the factory approved assembly procedure does not produce an assembly that meets the factory approved specs. This has apparently been true for decades, and nobody at the factory lifted a finger to do anything about it, thus leaving the dealers to homebrew solutions, and to absorb labor costs that they shouldn't have incurred.
- It was apparently "value engineered" by some dumbass bean counter who concentrated on part cost, hence the cross-pins instead of shoulders on the input shaft.

Mike Halloran
Pembroke Pines, FL, USA

 

[artbuc]

Mike, unfortunately, I believe your assessment is exactly right. But, as they say, it is what it is. What can I do to get the best performance out of this gearbox? Use Prussian Blue to measure contact pattern?

I believe the biggest problem I may have is that the input & output shafts may not be perpendicular and may not be in the same plane. I hope I can determine this from the tooth contact pattern.

 

[MikeHalloran]

If you don't have Prussian Blue, other substances may do as well:
- toothpaste
- Dykem or permanent Magic Marker
- Loctite PST

You won't get quite the same pattern as a hypoid will produce, but the pattern's position relative to the tip and root will tell you something about the adjustments you can make.

If the housing bores are misaligned, there's not much you can do. .... but because it's a die casting, _all_ of them would be misaligned the same way, so it's not likely to have gone uncorrected.



Mike Halloran
Pembroke Pines, FL, USA

 

[artbuc]

Mike, check this out. Ariens shipped 3 bad gearboxes in a row. The original on my new mower deck, the replacement box for me and a 3rd box the dealer had just ordered for a customer in FL. My dealer asked Ariens to ascertain if the die cast was out of spec. Ariens said they were not going to invest that kind of effort and they would obsolete the box first.

I found some contact patterns in the Arrow Gear catalogue for shafts which are not perpendicular or not in the same plane. I do not know how well they apply to straight bevel gears but I will give it a try.

I can't tell you how much I appreciate your help.

 

[artbuc]

This article should help.

 

[artbuc]

Arrow gear company has a series of video clips on YouTube. This one deals with analyzing contact patterns:

http://www.youtube.com/watch?v=Zi9PBZCgw-0

I got the new box from Ariens. It has more endplay than spec but it is much better than the earlier ones. It is also much quieter.

My local NAPA store carries Permatex Prussian Blue. I tried to look at the contact pattern by using the grease in the box. It doesn't work that well but I can see two things: (1) the pattern is very high and (2) I have hard lines. No question that the gears are not mating properly. I will get everything cleaned-up and use the Prussian blue.

Regarding nomenclature, Arrow and others refer to the gear set as "pinion and gear" or "pinion and ring gear". In my application, would the horizontal input gear be the "gear" and the vertical output gear be the "pinion"?

 

[MikeHalloran]

Since reducing gearsets are much more common than increasing gearsets, the input shaft would customarily be the "pinion", but in the case of a 1:1 gearset, the distinction is less clear.





Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

artbuc,

If you have access to some basic inspection equipment (surface plate, blocks, dial indicator and height gauge) you can check to see if the shafts/housing bores are coplanar and perpendicular. These problems are not things that can be fixed by shimming. Gear mounting distances and bearing clearances can be fixed with the appropriate shims.

As I mentioned previously, the .030 inch shaft end play seems way to large for your gearbox's tapered roller bearing sets. If your gearbox housing is diecast aluminum and your shafts are steel, then that shaft's axial clearance should be set close to zero at room temp, since the clearance will only increase at operating temps. The tapered roller bearings in your drawing look to have a contact angle around 20deg, which would equate to a radial runout of about .010 inch. And that amount of radial runout would not normally be acceptable at the gear mesh point.

Hope that helps.
Terry

 

[artbuc]

Thanks Terry. I have a dial indicator but not the other items. I may take the box to a local shop depending on what the contact pattern shows.

I disassembled the box last night. All of the red grease around the tapered bearings was black from what I assume to be Al particles. The bearing races for the vertical output shaft just fell out of the case. Don't know if the vibration caused this or if they never had a proper interference fit. Loctite has a product made specifically for cylinders (as opposed to threads) which I will use when I reassemble. This box has about 3-4 hours run time.

 

[artbuc]

Terry, I forgot to ask - why would Ariens specify 0.003-0.008" endplay when it should be close to zero? Do you think they are just allowing some extra play because the gears are not a high quality matched set?

The Loctite I mentioned was #609 or 620.

 

[artbuc]

Mike and Terry,

Tore the box down and cleaned everything up. Other than the output shaft bearing races being loose, it didn't look too bad although it shouldn't with just a few hours. I did a couple contact patterns at 0.006 & 0.013" BL. Horizontal shaft input gear has 22 teeth and vertical shaft output gear has 19 teeth. Output gear was hitting very low and teeth 1-8 rotated smoothly but 9-19 were binding. No wonder she was howling. Now the BIG experiment. Added 0.020" under the output gear. WOW! What a difference. Had to add more shims to the side cover to reopen BL. Got it to 0.010" with the shims I had. Since the spec is .005-0.015", I'll probably leave it there. With this set-up, the contact pattern looks good and the gears turn very smooth. I might raise the output gear a little more to see where the optimum point is, but I must be getting pretty close.

Of course, I won't know for sure until I put it under load, but the difference in the no load mode is dramatic.

Thanks again for your interest and help.