Torque Loss 1

[DocSkyHawk]

Good afternoon!

We have a motor that can produce 1500ft-lbs of torque at 500rpm. We need to be able to reach 2500rpms though. If we use a 5:1 gear ratio how much torque will be lost? Also, this would put a lot of stress on the gears, what type of metals would you use to produce them? Thank you everyone!

Doc

 

[MintJulep]

Power = torque x rpm x conversion factor = constant

 

[israelkk]

I you are using an increasing speed gear to reach the 2500rpms then your torque will be a least no more than 300 ft-lbs at 2500rpm.

Unless you will give more information on the space and weight limitations you have for the gearbox there is no point recommend materials. The larger the space and weight the lower strength materials you can use.

 

[GregLocock]

That's about the ratio you'll see in an automotive axle. That's about the power you'll see in an automotive axle.

So if push comes to shove I reckon an automotive diff would give you an idea of component sizes.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[DocSkyHawk]

250ft-lbs is our goal. If in fact we can keep the 300lbs then we are okay. As far as size and weight goes, something about the size of an automotive diff is okay. We were thinking in the lines of 12inx24inx8in. The main concern is a gear breaking. This is not an option. It must be able to run for hundreds of hours without failure.

 

[tygerdawg]

you need to hit the gearmotor company websites (SEW Eurodrive, Dodge, Nord, Falk, others) and get their free catalogs and engineering guides to educate yourself on this calculation process. The torque/HP equation is straightforward. Everybody here so far has missed including the 'efficiency' factor, for example. Or maybe even utilize their Applications Engineering departments to explore options.

Then you may consider using a Variable Frequency Drive which could allow you to run higher speeds out of the motor. After all of that, then you have to make sure that your mechanical elements can handle the torque loading and survive the requirements you state. THEN it may require a second iteration of calculation.

TygerDawg

 

[rb1957]

i'd have thought that the diff would be pretty darn reliable, if you want more margin then use a diff designed for more Hp, but that'll weigh more.

if reliability is that critical, how reliable is your motor, or the whole system ?

 

[DocSkyHawk]

Most of the system is designed for continuous use. However we will only be using it for two or three hour runs. But we can not afford a failure when in use. The main concern for failure with the gear is due to the high output of the motor. 1500ft-lbs is a bit more then what I would like to run on a mild steel. I will check out those sites as soon as I can. Thank you everyone for the help so far.

Also how did you come up with the 300lbs from the 1500 I stated? Is there a formula?

 

[israelkk]

Is it for a flight system or a ground or military system?

 

[NickE]

Gears are not made from mild steel. (Generally)

its an easy bit of math to caculate torque reduction from RPM increases. (or I always thought so, linear equations)

Hows an 85% efficiency factor sound?

So 1500ft-lbs @ 500rpm to 2500rpm is:

2500/500 = gear ratio ~1:5 increasing speed gear

Then at 100% efficiency the resulting torque would be 300ft-lbs (right, I mean if I gear up I loose torque at the same ratio as I gain speed) so by multiplying by .85 (frictional, flexure, fluid friction losses)
you get ~255ft-lbs available......

Seems easy to me, if you need an in line speed increase try a planitary type of gear, if you need to change the direction then you might change rotation direction and then you might run through two or three gears.

(I guessed at the efficiency, I really have no clue as to the available efficiencies.... I just think that this is not too hard of a problem and could be built out of standard gearing suppliers catalogs.....)

Oh, gears are generally made from alloy steels that may or may not be case hardened. (at least from what Ive seen. I'm sure that gears are made from everything including glass, but for your application thats what I would use, if weight wasn't a huge issue.

Nick
I love materials science!

 

[DocSkyHawk]

Thank you NickE! And this is a flight system.

 

[dvd]

As opposed to the existing 500 RPM motor and a speed increaser, you might want to consider a 150 HP, 3600 RPM motor running on a VFD. You may spend less on a different motor than the gearbox.

 

[MikeHalloran]

A flight system gearbox with a 1:5 speedup ratio and a huge input torque level is a very difficult undertaking, even when you know the exact nature of the prime mover and of the load. So far, we don't, so the advice offered here, however sincerely, is based entirely on guesswork. This back of the envelope stuff is not going to, er, fly.

Simple speed increasers produce extreme tooth loads and pinion bearing loads, even with modest ratios. Five to one is not a modest ratio in the world of speed increasers. Planetary gearsets nominally distribute the load, but require selective fit gears and probably controlled flexibility of the carrier in order to make that happen, and have the unfortunate habit of fragging themselves upon failure, so development is going to be expensive.

You need to find and hire an engineer who _really_ knows gears. This is not a two-week project, even for an expert. Don't even bother talking to the sales engineers for industrial speed reducers, except to identify the factory engineers you will want to steal from them.



Mike Halloran
Pembroke Pines, FL, USA

 

[israelkk]

First of all for flight systems the rules are different from commercial or industrial use. The materials has to be aircraft graded per MIL-HDBK-5. You should consult the appropriate authorities documentations. For example if it is for a commercial aircraft the rules and safety factors are different from combat aircraft. This will influence the materials and weight of the gearbox. You should consult the American Gear Manufacturers Association (AGMA) documentations. They have many guides for aerospace gearing design.

 

[GregLocock]

85% is a good guess for the efficiency by the way.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NickE]

Thanks greg......