Human Force to Turn a Bolt

This may be simplistic but here it goes anyway... I am designing a screw jack and need to have some idea of the typical amount of force a typical human (male 170 lbs) can comfortably exert on a wrench to turn a bolt.  By comfortably, I mean without bouncing or sharply jerking the wrench.  This force should be independant of the length of the wrench or size of bolt.  I realize human strength varies widely, that is why I indicated 'typical'.

Thanks in advance.

zakk,

I think a construction worker could apply easily a force of 100 lbs. provided he has enough room and a good point to react.   If he can not, he always can use a longer wrench.

AEF

why not just try out a couple of torque wrenches of different sizes.

zakk:  Assuming little body anchorage, I'd currently estimate a typical value of 170 N (38 lbf).

Using one hand, height at 22-42" the force is 66-83 lbf
2 hands would give a maximum of 165 lbf

Steven van Els
SAvanEls@cq-link.sr

I work with ADA (Americans with Disabilities) Standards, and there is one requirement that may help get you in some sort of ergonomics ballpark.  The requirement states that any manual operation of a vehicular wheelchair lift shall not exceed 50lbs of force.  50lbs isn't much to your normal male, but average females (~5'-4" ~110lbs) find it on the edge of "hard to move".

"Happy the Hare at morning for she is ignorant to the Hunter's waking thoughts."

I believe that there are two factors that you need to consider:
1.  Pain threashold -- the area of the wrech at the applied force will effect the tolerable pressure experienced by the operator.
2.  The proper use of a wrench involves torque, which implies a force couple -- both hands should be used to generate forces in opposite directions, otherwise your knuckles suffer.

DesignControl

Your local occupational health & safety office or workers compensation office should be able to provide you guidelines. Failing that, try to find yourself a good industrial ergonomics text or handbook, if there is such a thing.

Zakk,

In appendix S of the ASME Section VIII pressure vessel code there is an equation given that estimates the stress developed in a manually tightened bolt/stud, using standard wrenches as:  

Stress, psi = 45,000/[(bolt nominal diameter, inches)^0.5]

we can estimate that

Torque, in-lbf = CoF * Stress * bolt dia^3 * 0.785

where CoF is the coefficient of friction (I usually use 0.17)

we know that

Torque = Force, lbf * radius, inches

so given a standard wrench length you should be able to estimate the force.  Remember that this is all based on tightened torque.  This may be a little more torque than you want to allow over the full range of the lift, but it will give you some insight into a statistically determined upper limit.

Regards,

Gunnar    

When I used to design equipment for commercial and military ships, they usually specified a maximum of 50 lbf exerted by a human for normal operation of the equipment.  

I've seen this max. value for equipment in the industrial sector as well.