Helical Gearbox Overspeed

[Mumawalde]

Hello all...

Question, I have a 3 stage helical gearbox, ratio 200:1, with 13,000Nm max output torque.
I am trying to determine the maximum, no load, input speed allowable in such a gearbox. Under full load it is rated up to 3000 rpm.

Does anyone have any helpful rules of thumb or guidance I can dig further into?

Thanks!

 

[drawoh]

Mumawalde,

What does the manufacturer think?

--
JHG

 

[Mumawalde]

I am still waiting on an answer from the supplier. They have been slow to respond.

I was wondering if there are general rules regarding multi-stage helical gearboxes.

The actual case is backdriving where the only load is the motor itself, and acceleration is rather slow, but top speed is high.

 

[drawoh]

Mumawalde,

Do you know the armature inertia of your motor? Apply torque to the output shaft. Work out the acceleration torque on the motor shaft due to accelerating the armature. When I did this, I was very surprised. This may answer your question.

--
JHG

 

[Mumawalde]

Yes, I do know the armature inertia. When I ran the acceleration calculations, the gearbox was not heavily loaded. This is why I am stating that the case is low torque / high speed.

The gearbox has a 13,000 Nm rating, accelerating the motor is a fraction of this torque load.

 

[drawoh]

Mumawalde,

What is the maximum acceptable torque on the motor shaft? 200:1 is a heck of a speed increase.

--
JHG

 

[tbuelna]

Rolling element bearings typically require some minimum amount of load to prevent excessive skidding at the ball/roller and race surface contacts. This can be a problem with high bearing dN and low load operating conditions. If you have design data for any rolling element bearings used in the gearbox, you can check if they are operating with recommended minimum load at your max input speed.

http://www.skf.com/group/products/b...bearing-size/required_minimum_load/index.html

 

[Mumawalde]

Hello, the motors are quite strong, we are capable of outputting over 300 Nm. I am not worried about motor torque. Some calculations we have done have the acceleration of the motors very low (<3Nm) but the top speed very high.

I understand that there is a bearing issue, and potentially lubrication issue...

The bearing issue may be able to be solved with high speed bearings, and the lubrication issue could be solved by providing a spray of lubricant into the gear teeth.

Aside from these two problems / solutions, can you think of other issues I should be aware of.

 

[MikeHalloran]

I am still waiting on an answer from the supplier. They have been slow to respond.

I was wondering if there are general rules regarding multi-stage helical gearboxes.

The actual case is backdriving where the only load is the motor itself, and acceleration is rather slow, but top speed is high.

Were I your supplier, I would be slow to respond also.
General rule: 1:200 speedup is very hard on a gearbox.

I'm assuming that you haven't told your supplier exactly what load is backdriving, exactly what motor is being backdriven, or exactly what RPM constitutes 'high' in your world.

Don't hold your breath waiting for a useful response from the supplier, or from us, until you have supplied at least that minimal suite of necessary information.

​

Mike Halloran
Pembroke Pines, FL, USA

 

[Mumawalde]

Hi Mike,

I did provide full details to supplier, rpm vs. time and torque vs. time through the event. The rpm i provided is the motor rpm (high speed side of the gearbox) and torque is the torque required to accelerate the gearbox and motor rotating inertia. Torque is less than 4 Nm, and the rpm is up to 7000 rpm for about 5 seconds. The event could occur a few times a year.

Under load, I was given an OK to go to 3600 rpm. I was hoping by providing load info and specific curves I could get more accurate info / higher rpm.

Finally, what is the failure mode I should be expecting... i.e. reduced life, catastrophic bearing failure, tooth wear... etc...

Thanks All

 

[MikeHalloran]

what is the failure mode I should be expecting... i.e. reduced life, catastrophic bearing failure, tooth wear...

All of the above.

My friend Wayne spent much of WW2 rebuilding supercharger drives for B29 aircraft.. They used a planetary speed increaser; I think the ratio was around 1:15, but certainly not much higher. All the parts were machined to super precision standards, the best possible at the time.

Still, they had a substantial failure rate. Many/most of them just blew up, leaving no reusable parts. Wayne was kept busy salvaging what he could. If the housing survived, he could build a usable replacement, using a crate of brand new replacement gears, which he selectively fit at assembly, just as they were doing on the production line back home.

Since those days, gear manufacturers have made substantial improvements in tooth form generation, and auto transmission manufacturers have made great strides in load sharing with very subtle adjustments in flexibility of gear carrier structures and such.
... but you are asking a lot of even modern technology.

I'm wondering how much you could do about controlling the overspeed while/before it's happening, e.g. by a little DC injection into the motor coils while it's nominally 'off', and perhaps by direct mechanical braking of the inertial load without involving the gearbox at all.

You could get fantastic free help here, but there are obvious commercial reasons why you would not wish to share the details of your problems in a global public forum like this.

Given that, I suggest you engage in some private development with your gearbox supplier, using them as a partner, not an adversary.


Mike Halloran
Pembroke Pines, FL, USA

 

[drawoh]

Mumawalde,

My experience with high ratio drives was on a laser scanner. The gearmotor was a MicroMo unit with a gear ratio of something like 560:1. When you hold these gearmotors and you try to turn the output shaft, they do not budge. This one had a gear attached to it and it was mounted in a scanner with a plastic housing something like eight inches wide. I had no problem grabbing this and rotating it. I felt comfortable applying 100lb.in torque. I worked out the resulting acceleration of the motor armature and the torque on the motor shaft. The maximum allowable torque on the external housing worked out to something like 40lb.in. I asked if these things were failing, and they replied "yes".

No load at the output shaft does not mean no-load at the input, especially with a 200:1[&nbsp;]ratio.

What are you planning to do with this thing?

--
JHG