Spur gear wear

[Dorfer]

I have a pinnion of 21 teeth, 3 dia. pitch, 20 degree pressure angle, RC 28-32, 3" face width, climbing a rack which is RC 48-52 4130 alloy steel. The stroke on the lift is 40" 2x every 5 minutes is the travel at 10"/sec.
The four pinions carry approx. 9,000# each in the front and 6,000# each in the rear. There is a soft start with little shock loading.
The proplem is the pinnions and rack pitch lines were set tight with no backlash and resulted in considerable wear. The question is if I set the pitch lines correctly and increase the pinnion gear hardness to match the rack, will the gears live or are the pinnions now too brittle? I need to get to 10,000 hours of operation.

 

[diamondjim]

I would expect the life to increase
with a greater Rc hardness. The loads
do not look that heavy. What kind of
grease are you using and how many hours
are you getting now? How is the pinion
hardened? Carburized or Induction Hardened?
What is the core hardness of the pinion?
I would assume the back pinions saw very
little wear with respect to the front pinons.

 

[Dorfer]

The pinnions are auto lubed with Mobilux EP 0 Lithium multipurpose grease. They are currently pre-hardened 4130 28-32 RC. The wear on the teeth has been so severe the steel has been displaced off of the tooth sometimes up to .080-.100 deep, without shearing off the tooth? We plan to induction harden the gears to 48-52 RC. This is a 4 post rack system with certain misalignment, and out of level conditions of which could influence the gear engagement. Possibly .125" over twenty feet.

 

[diamondjim]

Does the pinion come back down with
a load? If not you might want to
consider a 50 percent long addendum
system. The pinion will be greatly
stronger and the wear should be much
less. I would use an ep2 grease with
the loads that you stated.

 

[rmw]

I am with diamondjim. I think you have lubrication failure. I certainly concur with changing to a #2 grease, but I question the lithium soap choice. #2 lithium is great in bearings, because it "flows" in and out of the bearing race and rollers as the surfaces heat up and cool down. What makes your lubricant "flow" back onto the rack or pinion gear tooth surface, once it has been "wiped" off with one trip up and down??? If you "wipe" the surface clean of lubricant, then you have metal to metal contact, and I would expect to find what you describe. I would like to know more about how you lube the gears, and more importanttly, how you "re-lube" them.

There are greases that are made not to flow, and therefore, not to "wipe" off the surface very easily. Lithium seems to me to have the wrong features for what you are trying to do.

I don't know which one to recommend to you, but I am sure there are other people in this forum who do. Whatever those other ones are, I just know I never wanted them in my bearing assemblies, because they weren't good there.

rmw

 

[Dorfer]

diamondjim, Please explain the long addendum and what is your opinion on the correct rack tooth form? Should we have a modified tooth that has a smoother transition with the gear? It appears the rack tooth, with a sharp corner is a virtual milling cutter wearing out the pinion teeth. The system applies auto lube between the guidance bushings and the round rack, but I suspect the actual rack teeth recieve little lubrication. We are sending a service man out to verify this. The moving platen houses the bushings and carries the drives for the gears as it travels up and down the stationary racks.

 

[diamondjim]

Basically a long addendum pinion would be
cut by withhold the cutter by .5 x the
addendum and the gear blank would increase
by 1.0 x the addendum. The basic center
distance would change also by .5 x the
addendum.
If your center distance is fixed, you might
try cutting 20 teeth on the same blank as
the 21 tooth blank.
Does the weight remain constant going up
and then down?
How are the 4 different pinions driven?
electric, hydraulic, or whatever?

 

[Dorfer]

Diamondjim, The center distance is fixed between the rack and pinion. There are no counterbalance systems to assist in lifting the weight. The four corners are driven electrically with one 60 hp motor, and cross shafting out of gear reducers. The service men in the field are claiming the racks in fact are not dry and are greased? I am pretty certainis the pinions are tight to the rack with no clearance, then the harder rack is milling the gear teeth away. Is there a modified rack tooth form than has a smoother transition with the gear tooth?

 

[diamondjim]

Dorfer,
You might want to radii the rack
teeth at their tips as they may be
digging into the pinion dedendums.
You might also want to increase the
length of the pinions to make them
stronger. Are the pinion and rack
teeth now the same length. You can
also have the rack teeth tip relieved
if you are having deflection problems.

 

[Frederick]

Have you taken a cross section of a failed gear? It could be that you have bad heat treat. Also, cryogenically treat the gears. The US Army found it doubles the life of helicopter gears.

 

[Gearhead]

Hi
With any kind of a failure you need to find the cause. An analytical check of the gears will tell you if you have a manufacturing problem. You should also take a look at your contact conditions.

 

[Dorfer]

Just as an update, upon installation of induction thru hardened gears, 48-52 RC, they quickly failed and cracked though the ringfedder bore which locks the gear to the shaft. Our gear calculations were performd by the vendor who provided the gears. We are now returning to the softer gears 28-32 RC without a clear solution to this problem. A case hardened gear is our next option.

 

[diamondjim]

Case Hardening may help.
You will still have the ductility
because of the softer core.
When you asked about greater life
with a higher hardness, I assumed
that you would Carburize or induction
harden to .045 deep or so to the
flank and root and not overlap at
the tips.