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View Full Version : Measuring force on a screw jack or jack post



Stephen Tashiro
07-12-2013, 4:09 AM
Is there a gauge or tool that can measure the load on a screw jack or jack post?

I'd think that one could calculate the force on a screw jack or jack post by measuring the torque used to screw it in place and knowing or measuring the mechanical advantage of the jack. I can handle the mathematics, but since such jacks are designed to be turned with rods, I'd have to improvise a way to hook a torque wrench into the tool that turns the jack. Do they make rod that has a socket or square drive holder on one end for this purpose?

I don't have specific application in mind, but when I use a screw jack as a more or less permanent support, I'm always curious how much weight it's really supporting because typically it supports only part of an structure like a floor beam so the load on the jack isn't obvious.

Lee Schierer
07-12-2013, 5:01 AM
I think the Machinery Handbook has formulas for calculating the force exerted by various thread forms based on applied torque. You can measure the torque with a spring scale attached to the end of the rod used to turn the screw and the length of the rod.

Jerome Stanek
07-12-2013, 7:13 AM
put the jack on a scale and raise it

Curt Harms
07-12-2013, 7:56 AM
put the jack on a scale and raise it

I was thinking a load cell but I suspect you'd need to be REALLY curious to pay the freight for such a gadget. Or maybe not, I have no idea about $.

John Coloccia
07-12-2013, 9:20 AM
I think the Machinery Handbook has formulas for calculating the force exerted by various thread forms based on applied torque. You can measure the torque with a spring scale attached to the end of the rod used to turn the screw and the length of the rod.

Bingo. That's exactly how I'd approach it.

Brian J. Williams
07-19-2013, 3:15 PM
For what it's worth, the relationship of measured torque to force (like bolt tension from a tightened bolt) can vary widely, from a clean/dry thread condition to a rusted condition to a lubricated condition. Case in point. If you have a clean A325 3/4" nut and bolt, torque testing will indicate one value. If that nut and bolt combination is rusted, the same torque force may only yield half the bolt tension as the dry thread case. Conversely, if one lubricates the clean bolt/nut combination with something like graphite and WD-40, the friction across the threads may be decreased to the point where, with minimal torque, you can exceed the tensile strength of the nut (or bolt) and tear the threads off the nut/bolt interface. Lee is correct that there are tables that approximate the torque-to-force relation ship, but most of those tables refer to a specific bolt/nut condition (such as clean/dry), and variation from that condition can nullify the results. If it's something critical you're trying to measure, call a local testing laboratory and ask them if they can measure the load/torque relationship in one of their compression testing frames.

Art Mann
07-21-2013, 10:25 AM
Do you have an estimate of the load? There are devices that are placed between the jack and the load that measure the force directly by way of hydraulic pressure. For example, here is one that will measure tongue load up to 2000 pounds on a travel trailer.

http://www.etrailer.com/Tools/Sherline/5780.html?feed=npn&gclid=CLGpiMXjwLgCFUPl7Aodu3AAJg

I am not endorsing this particular brand as I have never used it. I am just using it as an example.

I agree with Bryan that the variation in friction forces on the screw are so great that you will not get a very accurate measurement using torque. You might be able to measure a rough estimate to within 20%.