progressive HP rating in multi reduction gearbox 1

[Tmoose]

100 HP motor driving a mixer through a dual reduction gearbox. First reduction via helical gears. Second reduction is via spiral bevels. The gearbox supplier designed the spiral bevels using 85 input HP, claiming that is what gets passed on thru the helicals.

Is this commonly done?

thanks

Dan T

 

[MikeHalloran]

I've never _noticed_ anything like that happening.
Does the end product have to fly, or is there some other reason to design on the razor's edge?
How long has the supplier been in business? Maybe they know something I don't know, but I didn't think helicals were that lossy.




Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

Tmoose,

A normal helical gear mesh should not lose more than about 2%, even with poor lubrication. To be honest, a typical single helical gear mesh with 15% losses would likely fail very rapidly in scoring. So I can't believe that number is correct.

It may be possible that you misunderstood what the supplier meant, or that the supplier spoke incorrectly. Gear teeth are commonly designed for fatigue loads that are less than 100% of the input power, since the gearbox does not tend to operate at 100% power for 100% of life. Your supplier may have meant that the spiral bevel gearset was designed for a max continuous input power of 85HP, which would seem quite reasonable.

Good luck.
Terry

 

[gearcutter]

Are these the same spiral bevels you posted pictures of a few weeks back.
If they are; I can see an interesting scenario beginning to develop here.

What is the required duty cycle?


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[Tmoose]

Hi Ron,

the scenario is on chapter 16.
Duty cycle = REAL close to 100% load, all day, every day, except for an outage for plant maintenance every now and then.

Dan T

 

[gearcutter]

Hi Dan,

85HP seems too low to me, considering the 100% duty cycle.
Electric motors can develop significantly higher power values than the name plate value.

Without knowing the full details of your set; I would have started at 100HP and gone up from there after applying application/service and safety factors.
The damage suggests that the gears are not fit for purpose and perhaps what you have discovered is why.

Seems like now might be a good time to get an independent party to validate the design.
IMO this should always be the first step before ordering a gearbox.
My experience has been to never ultimately trust what a gearbox supplier is telling you will do the job.
Most of the failure analysis I do is on gearboxes which are relatively new and have been found to be not fit for the application.



Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[tbuelna]

Gears can be limited by torque or power. In very general terms, tooth bending and contact stress are more a function of torque, while scoring is more a function of power. Helicals or spurs would more likely be limited by tooth bending or contact stress (ie. higher efficiency but weaker teeth). Spiral bevels, on the other hand, are likely to be limited by scoring rather than contact or bending. Due to their inherent greater relative contact sliding and greater face contact ratio (ie. lower efficiency but stronger teeth).

Unless the shaft speeds are low, it's quite normal for those spiral bevel gears to be power limited.

Terry

 

[gearcutter]

Terry,
I agree with you comments about power vs torque.
This set of spiral bevels is also the set mentioned in a previous post where Timoose mentions the details. Speed is low suggesting high torque.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[Spud6414]

I went and looked at the pics in the other thread of yours, I would agree with gearcutter.

I would also ask, is this an Amarillo gearbox?

 

[Tmoose]

100 HP @ ~100 rpm = ~5252 lb-ft before applying discount coupons.

We've had a few break ring gear teeth. One ran less than two weeks after having a brand new Ring and pinion installed during a factory rebuild. Some others showed levels of localized spalling when disassembled for other reasons. Mobilgear 600 XP 220 should be up to the task.

So I'd say we've repeatedly exceeded the gears limits for bending strength and contact stress. Unfortunately it looks like the relative influences of manufacturing vs design (with subset of manufacturer's predicted operating loads, plus wildcards) won't be answered any time soon.

 

[MikeHalloran]

It seems clear from here that you need more gearbox or less mixer torque.

Was something in the mixer changed after the gearbox was selected?



Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

We buy the device (motor, gearbox, base with legs, whirly thing, couplings, seals) as an assembled commodity. The impeller design etc are all by the experienced supplier to create our broadly defined flow requirements. The insides of the reducer were a "black box" until fairly recently.

Yes, the LS gear ratio was changed on the twelve units at this first installation when the motors were drawing excessive amps. The installations that followed got smaller impellers in place of a ratio change. Reports are They have not suffered gear breakage, spalling, etc, so I'm personally keeping "bad bucket o'gears" on the list of suspects. This installation's (reportedly unique) Visible low frequency vibration and fairly strong blade pass have captured most folks attention. I'd like to measure instantaneous power to see if the 100 HP is ~steady, or has a big 3X component. I hesitate to say or think this level of vibration is "causing" a problem for the gearbox.