Shaft Wear

In our robot drivetrain we have a set of 20 DP (3/8" face width) gears with a 2-speed shifting mechanism. The gears (high grade steel, not hardened) are bored and reamed to .750" and have 4 semicircle cutouts 90 degrees apart. Each semicircle is approximately .125" deep. The gears ride on a stainless steel shaft with 2 sets of holes 90 degrees apart around the shaft and the center of the shaft bored out approximately 1/2". Ball bearings ride in each hole on the shaft and a plunger is pulled back and forth inside the shaft. To engage a particular speed, the plunger is pulled into alignment with one set of holes and ball bearings (1/4" dia stainless steel, precision ground) are pushed out to mesh with the semicircles cut into the bore of the gear. The other gear that is not engaged free rotates on the shaft.
The problem we have been having is that on one of the gearboxes something has caused wear on the shaft which leads to the non-engaged gear unable to freely rotate. This leads to the equivalent of both speeds being engaged at the same time (not good) and effectively stalls the motor. We have taken apart the gearbox and replaced the shaft, gears, and bearings with spares, and made sure that both the gears and the shaft are clean and lightly lubricated. The gearbox is enclosed so there is no foreign material that could be getting into the gearboxes. The wear seems to be build up of metal from the gears onto the shaft. This is not a flaw in gearbox design, as the system worked for many hours of competition and testing earlier in the season. In addition, one gearbox has never had this problem, and the other gearbox has done this twice in two days.
I'd appreciate help from anyone who might have an idea as to why this wear is ocurring and what I might be able to do to prevent this from happening. We have a limited number of shafts and they are rather difficult to machine and so we don't want this happening if we can avoid it.
You can see the shaft, ball bearings, and gears in http://engineer.la.mvla.net/robotics/inventor/Pictures/Gearbox/DSC00139.JPG
I will try to post a link to a magnified picture of the wear on the shaft itself in a few days.
Thanks in advance, Michael Los Altos High School Robotics Team "Eagle Strike" Los Altos, CA www.lahsrobotics.org snipped-for-privacy@gmail.com

Stainless galls and is as you say rather difficult to machine. Try some non stainless steel shafting. If you are going to use the shafts you have already made, use some 600 grit or finer sandpaper around a dowel and get the hole in the gears as smooth as possible. Use synthetic oil ( mobile one ) or krytox grease and I think you will be fine.
Dan
woodworker88 wrote:
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snipped-for-privacy@gmail.com says...
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What you're seeing is galling, not wear, and is a materials rather than a design problem. The best solution would be to replace one of the sliding components with a better bearing material. If the shaft is a diificult part to make, I'd consider a bronze insert in the bore of the gear. Regular SAE 660 bearing bronze is easy to get hold of and easy to machine if the loads are low. One step up would be a phosphor bronze. The proper aluminum or manganese bronze would be at least as strong as the gear.
Enter "bronze" in the search window at http://www.mcmaster.com/ and you'll get an overview of the common bronzes. More detailed info is available at Matweb.
http://www.matweb.com
Ned Simmons

Some never sieze lube might get you through the day until you can replace the shafting with something that doesn't cough up galls. Work the smooth parts with 600grit autobody sandpaper or even finer. A wood dowell with a slot makes a halfway decent flexhone.

As Ned said, gauling is a common problem in sliding applications and is usually corrected by a change in material. If you need to remake the shaft strongly consider one of the graphitic tool steels, such as O-6 or A-10. These steels contain free graphite which acts as a lubricant is sliding applications. I have used these steels in the past and have had good success. YMMV.
John Normile
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Thanks to all who responded. The gearbox isn't totally enclosed, so I can't use an oil bath, but I am definitely going to try and see what I can do to add lubrication. Clearly a change in future materials is in order. I have had problems with surface rust on regular steel shafts that interferes with movement, but I will see what I can find in terms of bushings, etc and other possible shafting materials. Thanks again for the advice, Michael

woodworker88 wrote:
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Other than proper lubrication, you can try Nitronic 60 material for some of the mating parts. Virtually all stainless steel material galls at 2000 PSI..Nitronic 60 parts mating with either other nitronic 60 or 300 series galls at about 50,000 PSI. Nitronic 60 is a stainless material that has about the same corrosion resistance and properties as T304 stainless.
Even if you get past the galling, you may have wear problems over time. Stainless is a very bad wear material and acts pretty soft when used as wear parts. Precipitation hardened grades of stainless (17-4 ph for instance) will improve wearability but will NOT improve the threshold galling stress: It'll gall at 2000 PSI just like the unhardened 300 series stainless.
Lubrication is the real answer though...cheap and effective.
Koz

woodworker88 wrote:
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Why use stainless steel at all in such an application? You're not running in salt water, right? If it's a properly designed unit, it's going to have gear oil in there, so it's not going to rust. Stainless steel has an annoying tendancy to gall and it's hard to machine. Get some decent carbon steel shafting in there, either carburized or case-hardened if you're going to have bits rotating on it, and use some decent gear oil with high-pressure additives. Bush your gears and bearings with bronze or cast iron and you'll not have the same problems. Cast iron and steel will work for bearing and shaft, steel and steel, no. Check the coefficient of friction tables.
Stan