Wikipedia:Reference desk/Archives/Science/2021 July 23

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July 23

Conversion factor for torque

Repost as suggested from the Mathematics section.

Hi all, I'm trying to find the conversion factor (or constant) to work out the torque of some old engines and their gearboxes. Torque is given in Kilopondmetres (abbreviated in my sources as mkg or kpm), and power is stated in metric horsepower (PS). I realise that max. torque is often reached at lower revs than max. power.
For example:

• Torque (lb.ft) = 5,252 * power (hp) / speed (rpm)
• Torque (N.m) = 9.5488 * power (kW) / speed (rpm)
• Torque (kpm) = X * power (PS) / speed (rpm) — what is X? And if you're feeling charitable, why? I understand that 5252 ≃ 33,000 ÷ 2π.

Cheers, MinorProphet (talk) 02:15, 23 July 2021 (UTC)

GNU Units does this kind of thing automatically:

You have: metrichorsepower/rpm
You want: kpm
* 716.19724
/ 0.0013962634
That is, X=716.19724. 2601:648:8202:350:0:0:0:2B99 (talk) 03:27, 23 July 2021 (UTC)

Wow, that's fantastic! Thanks very much for your help. My PC's hard drive has been developing bad sectors and is likely to die soon, so installing another app wouldn't be good idea: otherwise I would have tried GNU Units. I hunted around a bit for the database/list of supported units, but couldn't find it. Best wishes, MinorProphet (talk) 04:22, 23 July 2021 (UTC)

Partially

Resolved

(MinorProphet (talk) 18:32, 23 July 2021 (UTC))

As an addendum, it is very easy to work out the torque/ angular velocity/ power relationship. For example the 5252 number is not magic, it is embedded in the definition of a hp. Greglocock (talk) 10:24, 23 July 2021 (UTC)

Thanks for your reply. As I said in my OP, I am aware of that: but what is 9.5488 in the definition of a KW, and 716.19724 as regards a metric horsepower (= 735.49875 W)? I imagine they are something divided by 2π, but what? You said "it is very easy to work out": for me it is very easy to speak French and German fluently, and Russian and Italian with a decent accent at least; but the mere mention of angular velocity and radians per second brings down the safety curtains of my mind. Cheers, >MinorProphet (talk) 15:39, 23 July 2021 (UTC)

1 minute = 60 s, and 1 PS = 75 kgf⋅m/s. The numeric value of 60 × 75 / (2π) is equal to 716.19724... I suspect that has something to do with it.  --Lambiam 19:55, 23 July 2021 (UTC)

Aah, genuine enlightenment from an unsuspected quarter. Top marks, go to the top of the class. Y MinorProphet (talk) 21:44, 23 July 2021 (UTC)