Is there any reason a forged replacement gear would cause problems to surrounded cast gears?

[MechE_2694]

I've got a very old large gearbox (50+ years old) in which the bull gear (~26") has a massive crack. The gear company that is working the rebuild effort is suggesting a new forged gear out of a different metal to replace it. They've stated that it will have a similar gear hardness to the existing gear, but before we pull the trigger I want to make sure there's no risk of the new forged (presumably much stronger) gear causing excessive wear on the surrounding original casted gears? The vendor has assured us this is a common issue that they solve with old gearboxes, but given the price tag I want to have more backup for upper management than just "the vendor told me so".

Is there any resources that can back up why or why not, like a standard or case study?

TIA

 

[mfgenggear]

OP
Was there a failure analysis of why the bull gear
Had a failure.
Generally the pinion gears will have better metlab properties. Because it has more cycles and is prone to wear.
A bull gear metlab properties will not have to have a higher hardness values.

Send out the bull gear for a metlab review for
Core hardness, gear teeth properties, if case harding present. Gear teeth surface hardness.
To verify the exact original design criteria.

 

[mfgenggear]

OP
Forging for this application from the limited information is over design. Take the cast gears
And have micro hardness test on the faces of the gear teeth , and not the gear flanks involute areas.
There is a possibility the cast gears have case hardening. Ask the metlab if the can or have the ability to NDT the teeth to verify case hardening.
And evaluate the hardness with out damage.
This will dictate the hardness of the bull gear.

 

[MechE_2694]

OP
Was there a failure analysis of why the bull gear
Had a failure.
Generally the pinion gears will have better metlab properties. Because it has more cycles and is prone to wear.
A bull gear metlab properties will not have to have a higher hardness values.

Send out the bull gear for a metlab review for
Core hardness, gear teeth properties, if case harding present. Gear teeth surface hardness.
To verify the exact original design criteria.

This was the result of bearing failure. We sent it off to be inspected and rebuilt, but discovered the crack in the bull gear. The other components have been visually inspected and MPI tested and confirmed to be in good condition.

We're currently exploring an option we might have of sending a gear from an unused spare gear which is probably the ideal solution, but I was just hoping to get an answer regarding the forged gear being intermixed with the existing cast gears. I didn't think there would be any issue as long as the hardness was the same, but also I haven't had much gear experience since graduating.

 

[3DDave]

More a question on the surface finish and lubrication than the gear material. If it is more wear resistant then it will provide a more reliable interaction with the mating gear. The worst case will happen regardless of material if the mating gears are worn in a way that the new gear doesn't mesh well; it may be worthwhile to have the gears inspected and possibly reground to ensure involute action is taking place and that lubrication is working.

 

[mfgenggear]

More a question on the surface finish and lubrication than the gear material. If it is more wear resistant then it will provide a more reliable interaction with the mating gear. The worst case will happen regardless of material if the mating gears are worn in a way that the new gear doesn't mesh well; it may be worthwhile to have the gears inspected and possibly reground to ensure involute action is taking place and that lubrication is working.

that is a great point if the gears soft the gear meshing will or must be worn in at low RPM.
the surface finish of the bull gear must be equivalent or better than the original.
what we don't know what is the torque and rpm requirements of the gear box.
and what caused the gear failure.
and why bull gear failed before the cast
gears which is rare.
pS:
in my line of work, gears are coated with silver or copper plating for break in.
but generally gear boxes need a break in in high torque and rpm. it allows the gear teeth to wear in evenly. as at high rpm gear teeth with distort. causing uneven wear.
causes premature wear.
case in point . an after market gear replace ment contracted my company to manufacture a high torque high rpm sungear. it had the incorrect involute and surface finish it caused premature failure.

 

[mfgenggear]

This was the result of bearing failure. We sent it off to be inspected and rebuilt, but discovered the crack in the bull gear. The other components have been visually inspected and MPI tested and confirmed to be in good condition.

We're currently exploring an option we might have of sending a gear from an unused spare gear which is probably the ideal solution, but I was just hoping to get an answer regarding the forged gear being intermixed with the existing cast gears. I didn't think there would be any issue as long as the hardness was the same, but also I haven't had much gear experience since graduating.

my bad did not read post correctly.
do have the recorded harness difference.

also I forgot to mention because of size,
bar normally requires a minimum length from the supplier. thus the cost may be more significant than single plus a setup part.

I will repost what I said earlier. depending on the gear ratio and mating gear it will significantly rotate more than the bull gear and will cause significant premature wear.

 

[MechE_2694]

More a question on the surface finish and lubrication than the gear material. If it is more wear resistant then it will provide a more reliable interaction with the mating gear. The worst case will happen regardless of material if the mating gears are worn in a way that the new gear doesn't mesh well; it may be worthwhile to have the gears inspected and possibly reground to ensure involute action is taking place and that lubrication is working.

The vendor is proposing to grind the teeth to ensure good contact along with MPT inspection of all gears, so it sounds like we're going down the right path. They also mentioned that the meshing of the gears, hardness of the gears, and lubrication were all more important aspects, so that checks out. So assuming all gears are in good condition and the vendor adequately mates the gears, matches similar gear hardness, and ensures proper lubrication, there's no reason I should be concerned about a forged gear being intermixed with the other cast gears?

*Also just to clarify above, the original bull gear was also a cast gear. I re-read my post and I think it may have read that the bull gear was originally forged, but all original gears are cast, so the new forged gear will be used alongside the existing cast gears that have already been MPT tested and inspected.

 

[mfgenggear]

The vendor is proposing to grind the teeth to ensure good contact along with MPT inspection of all gears, so it sounds like we're going down the right path. They also mentioned that the meshing of the gears, hardness of the gears, and lubrication were all more important aspects, so that checks out. So assuming all gears are in good condition and the vendor adequately mates the gears, matches similar gear hardness, and ensures proper lubrication, there's no reason I should be concerned about a forged gear being intermixed with the other cast gears?

*Also just to clarify above, the original bull gear was also a cast gear. I re-read my post and I think it may have read that the bull gear was originally forged, but all original gears are cast, so the new forged gear will be used alongside the existing cast gears that have already been MPT tested and inspected.

Was the involute tested and recorded as well.
How was the hardness tested on the teeth.
And how was it tested.
Major diameter and tif, root diameter, was it a straight involute, or modified. What was the root radius, and what was the surface finish on tooth flanks. Was the measurement over wires inspected. How much back lash is required.
Cast gears are the worst . Amazed it lasted 50 years. Now how's the bearings are they being replaced as well. How was the runout of the other gears, the shafts have any runout.
What was the failure analysis results.