Torque Rating of Gearbox

[navi26]

This seems to be a pretty basic question, but I have probably been over-thinking it. I am trying to clarify some torque issues with one of our gearboxes. Is there an equation for torque that the manufactures use to rate their gearboxes? I know that Power=Torque*speed or HP=torque*rpm/constant. For couplings, Torque=63000*Pnormal*SF/Nnormal (already has constant included?). I was wandering if this formula is applicable for gearboxes also? I am guessing that the gearbox manufacturers have the same gearbox for a range of power and they only change the S.F. to have an accurate nameplate. We have changed the driver power and I wanted to make a quick check to see what the gearbox is capable of handling. If I used a new gearbox I will probably have to put a new foundation in.

Background info:

Gearbox ratings
32hp
In RPM: 3402
Out RPM: 808
Ratio: 4.211
S.F.: 1.46

Steam Turbine Driver

 

[tbuelna]

There are many things to consider when rating gearboxes. Torque is typically the most relevant. The gears see forces at the mesh pitch line due to torque. The most common limiting factors for gears are tooth bending, scoring and pitting due to contact stress. A proper evaluation will require you to assess the gears, bearings, shafts, splines, lubricant, etc., for suitability at the new loads and speeds.

No, unfortunately, there is no "easy" method to do the job properly.

 

[Javier72]

As it is said by tbuelna, to caculate a gearbox is not so easy. Only to calculate a pair of gears, it takes a long time to optimize every needed parameter. The torque capability is going to be influenced by the teeth number, the pitch, the hob advance, the profile type, the clearance between the teeth,the machining accurance, the contact angle,... After optimize this data, you have to select the adequate material for each piece, treatment, grease, ... and then, you could estimate your torque resistance in perfect manufacturing conditions, not in real conditions. Of course, it is not so easy, and it takes a lot of time for each gear. After having your gears calculated, as tbuelna said, then you have to calculate the shafts, bearings,...

Then, if you want to have a well calculated gearbox,it is necesary to be working with it a very very long time, and there is not an easy way to do it

 

[navi26]

Well, I guess I am kind of puzzled why we are just seeing problems now. Fifteen or so years ago they changed the hp of the driver to 45hp and didn't change the speed. I'll have to look at how they operate this to see if the fan is being used often or not. I will probably just have the manufacturer check to see if the gear box can handle our current conditions. Thanks for all the help.

 

[PeterCharles]

Go to the gearbox manufacturer, give him the serial number of the gearbox and ask him what the rating of the gearbox is at 3402 rev/min. He'll have all the design data and can tell you the maximum power, the torque the gearbox can take, and its thermal rating.

 

[BobM3]

Going from 32 HP to 45 HP is a significant increase in power (and torque, if the speeds are the same). But 15 years is a long time for the problem to show up. Do you run continously? Has the ambient temperature changed recently? Has the oil change frequency changed? The type of oil? The nature of the load?

 

[plasgears]

Asking a gearbox to sustain significantly more power has the following implications:
- higher tooth loads and slliding components;
- higher bearing and bearing support loads;
- higher gear pin or trunnion loads;
- higher mounting loads on the mounting means;
- higher heat load on the lube system whether grease or oil pressure.

The gearbox may be able to stand up to the new conditions, but you have to prove it. Run an exhaustive cyclic testing program.

Along similar lines, how do you test a product that requires continuous duty for a very long time? Challenge it with higher loads, degraded lube, less cooling, etc. The exact proportions of the challenges are according to your judgement as agreed by your peers.

 

[dimjim]

If the loads have not increased and the speed remains
constant, I think you would expect the heavier hp motor
to last longer.
If the speeds remain constant and the loads increased
your contact stresses would be higher. You might need
a better lubricant to handle the higher contact stresses.

If the failure is the result of higher contact stresses
having a dynamic effect on life, I think you would see
only 90 percent of the life of the gears in the gearbox
and this would assume the same gears as in the 32 hp
gearbox but maybe of a better material and quality.
There are so many variables that would have allowed the
increase in rated power or torque.

 

[navi26]

A little more information for all of you. At that speed the mfg. said it is good for ~50hp and ~80hp thermally. The turbine and gearbox is seldomly loaded. They usually are on a slow roll and are engaged when/if we lose power. With the subsequent drop in speed, the clutch will engage and switch the load from the motor to the turbine. We run Royal Purple synthetic, but I can't recall the grade at this time.

I think, but I will need to ask around, that the shafts on the gearbox failed when the clutch engaged and transfered the load. We don't have any run-up data or useful indicators on the turbine to see the torque during run-up.

 

[tbuelna]

As far as tooth bending and contact stresses are concerned, you can probably run higher torque through the gearbox, you'll just have reduced life. Most (long life) commercial gearboxes are conservatively rated for bending and contact.

However, if you do not have adequate scoring margin, your gearbox will fail in a matter of minutes. Scoring function can be slightly improved with a better grade of lubricant and by putting a better finish on the gear teeth. Both of these can be done to your existing gearbox without too much effort.

Also you haven't mentioned what type of gears these are. I'm assuming they're spurs or helicals?