Lifting Force Created by 7/8"-14UNC "Jack Screw"

How about this?:

http://www.roymech.co.uk/Useful_Tables/Cams_Springs/Power_Screws_1.html

So, the basic idea is to use work in = work out (optimal friction and all).

W_in = Tθ = 150 ft-lb x 2π for one revolution

= 940 ft-lb

W_out = Fd

F = W_out/d = 940 ft-lb/0.006 ft for the advancement in one rev.

F =160 kips in a perfect world

Hi TVP45

But we don't live in a perfect world! 160 kip gives a bolt stress 139 ton/in², which will likely break it. Usual torque calculation assumes dry threads and only 10% of applied torque produces tensile stress in the bolt, the rest overcomes friction. For lubricated threads it will be higher, I'd guess 20 - 25%, giving max 40 kip. Whether this is OK depends on the bolt strength grade. Bolt data I have shows 190 ft.lb to give 11.3 ton/in², for a low strength grade.

Not clear from Bentarrow's OP whether for his application a safe figure is a high or low estimate. The same formula applies near enough for pushing as the more normal bolt tightening, as far as I can see. It's not an exact science either way.

Is it possible to measure the bolt strain with a micrometer? If so, load can be calculated from the elastic modulus. Did this some years ago to settle a dispute about bolt loads being applied to a big flange.

Guest's link is interesting but applies to lifting threads, not ordinary bolts. For bolts, all the complicated calcs are rolled up into the 10% figure!

Cheers......Codey

Hey Codey,

Agreed. I was just giving him the general idea without all the derating factors thrown in. I don't know anything about his application, his materials, etc.

Happy Thanksgiving!

Hi again TVP45

Thanks for the season's greetings. We don't do much with Thanksgiving here in the UK, more Diwali and Ramadan these days! But Christmas is coming.

Cheers.......Codey

Well, you could be thankful all us colonials are not coming home to visit the ancestral huts for the holidays.

Jack bolts will often break cast iron.This is from 40 years of experience.The more stress,no matter the reason,the better the chance.This is why most jack bolts that will be heavily stressed are not threaded into cast iron.The ones threaded by the manufacture into cast iron for jacking often break out or strip.For a example.The heads on large "Viking" Gear pumps are bad about this.Viking has now been making them for close to 100 years (1911) and it is still a problem.They did it to me for 36 years as they were threaded to "jack"the heads off.Great idea,until they strip or break out!

Bolts,studs & cast iron strengh,thread clearances,smoothness of the threads,force applied all leave so many varables that each individual one has a difference.The safe way if possible is a stud or bolt that is not threaded into the cast iron.Even then you can often hear the sound of cast iron breaking under stress with that awful snapping sound like no other.Steel flat washers or plates under the nuts and against the cast iron will distribute the force and often keep the cast iron safe.The threads on the bolts and studs can always be far stronger than cast iron if you use the proper ones.Some of the strongest metal known is used in some studs,nuts and bolts.Alfred

The force generated in the threads is the same whether jacking or clamping the only difference then is in the tension verses compression strength of the material of the bolt, or the bursting strength of the nut.

Since the strength of steel in compression is greater than that of steel in tension a steel bolt in a cast iron nut will typically snap the bolt in tension while it may burst the nut in compression. A steel bolt in a steel nut in compression should fail the threads or the jack while it should fail the bolt in tension.

I hope this helps you understand your situation better.

Back to Codey's point. I think the reason you don't see this stated in a very straightforward manner (other than in textbooks where, they say "assume 10% friction losses" or similarly) is that the firctional losses are so very variable.

If this is a specially made jacking screw with precision threads (7/8" would not be ground, I think) and well lubricated, I think it safe to say you could expect 40% nominal efficiency, with the occasional +/-30% variation.

But, I would measure it. I thought maybe I'd set up my gear puller (which is a pretty good jack) and measure it today, but I got up this morning to find the downstairs commode leaking and the upstairs sink drain not working (we had the vandals and goths in for the holidays, so??). So, my day is now a plumbing exercise rather than one of measuring jacks.

Hello TVP45

It's not just textbooks, bolt suppliers use it to determine recommended torque. Assuming thread pitch = dia/8 and efficiency = 10%, can derive formula

Bolt load (N) = 5000*torque(Nm)/dia(mm)

Cheers.......Codey

Some years ago, I got called down to the production floor where they were having trouble with "C" cores being crushed in a clamp arrangement that had always worked before. At first everybody assumed we had an epoxy problem, so we called in the vendor, ran tests, got new samples, etc, until I actually watched the assembler. He was taking the clamp bolts (which had a torque spec), carefully dabbing some MoS₂ on the threads and torquing them down. So, we had to add a note to the process sheet and drawing: "Do not lubricate".

Poor guy no doubt thought he was working beyond the call of duty!

BTW, what is a "C" core?

Take a strip of magnetic core material, for example 5 cm wide x 0.02 cm thick silicon steel, and wind it on a rectangular form (with rounded corners) using an epoxy varnish to hold it together. Wind it to maybe 2 cm thickness. Bake it to cure. Cut it to form two "C"s. You can put them together with just shipping straps when assembling.

Interesting, thanks