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poor mans torque multiplier?

W. Carter Marcy

Aluminum
Joined
Dec 15, 2002
Location
Troy, NY, USA
A customer of mine has asked me to make what could best be described as a crows foot wrench with an extension between the square drive and the wrench opening. In other words, it looks like a standard combination wrench, but instead of the box end it has a square hole for a torque wrench to fit into. The customers intent is to use the torque wrench to regulate the amount of torque at the business end of the extension wrench. I believe that the intent is not to make a torque multiplier, but rather to adjust a nut that is only accessible in this way.

For the purpose of discussion let’s say this wrench is 24 inches from center of wrench opening to center of square drive. It will be used with the torque wrench in line with the axis of the extension wrench. The requirement is to apply 1600 ft/lbs of torque to the nut.

My questions are whether or not this is as simple as increasing the length of a lever, or is it more complicated? Does the fact that the torque wrench measures torque, not lbs force on the end of the handle make a difference. Does the type of torque wrench used matter (beam with pointer vs. micrometer style)? What torque value would need to be applied with the torque wrench to achieve the 1600 ft/lbs at the nut, using the extension?

My interest in the theory behind all of this purely academic. I will happily make what the customer has asked for, regardless of whether or not his theory is sound or not, as I was not asked to offer my opinion on the matter. I am interested to hear what other folks think.
 
Fun question! I can show you the math if you want, but here's what to have in your pocket - The torque you get from using the extension depends on the length of the torque wrench, so you can't really call the extension a "10 x multiplier". It might be 10x, it might be 1.01x, depends on the wrench. It also depends on if the extension is sticking dead straight out from the wrench, or at an angle.

It does not matter beam clicker etc to first order. If you're getting fancy then the wrench deflection has to be figured in, but torquing is not very accurate in any case so in normal life you wouldn't figure in the deflection.

He's going to need one heck of a torque wrench to get to 1600 fp with that. I have an 800 fp wrench that's like 6 feet long. You wouldn't hit 1600 using your extension on it.

What would make for a bit of head scratching is using an actual torque multiplier with that extension. I might have to make a drawing to sort that one out...
 
kurcules,

I might want to see the math later, but for now please tell me why the length of the torque wrench matters. If it registers torque isn't that all you need to know? And yes, let's assume the extension is sticking straight out. I guess that the torque at the drive vs lbs force on the end of the torque wrench is what puzzles me. For instance if you applied a strictly torque motion to the square drive hole of the extension (like an impact gun for instance) it is different than a torque wrench, right?
 
...what kurcules said...
If you make a 24" extension, and your torque wrench is 24", then you have a 2X multiplier. And this assumes everything is in straight line.
1600 ft/lbs is a lot. You will need huge mechanical advantage to get that from your biceps.
 
You are correct in thinking the torque is different with any deviation/extension from the centerline of the square drive on the torque. There is a formula for calculating this accurately, I am certain the applied torque will be greater than the value read/set on the torque wrench the farther you extend the center lines between the driver and driven points.
I don't think the length has any bearing on the formula for this, only the offset difference between the driver and driven points, let the fun begin.
Dan
 
kurcules,

I might want to see the math later, but for now please tell me why the length of the torque wrench matters. If it registers torque isn't that all you need to know?

Because the torque reading on the wrench is not on the nut in this case. It is on a point in space 24" from the nut.
 
Yes torque is torque, but it's hard to work this problem directly on the torque because you have both the torque, plus the force being applied to the handle that's pushing "sideways" on your extension. But anyway, lets see if I can show why wrench length matters...

Figure you apply 100 fp to a bolt. If the wrench is 1 ft long, that's 100 lbs of force. 2 foot long, 50 pounds of force, and so on.

So depending on the length of the wrench, different forces get the same torque.

Now you stick on the extension. Lets say the wrench was 1 foot long, so you are applying 100 pounds of force for a 100 fp reading on the wrench. But that 100 pounds is actually 3 feet from the bolt, thanks to the extension. So the bolt sees 300 fp of torque.

Now lets take the same 100 fp wrench reading, but on a 4 foot wrench. That's 25 pounds on the end of the handle. Add the extension, now you have 25 pounds 6 feet from the bolt, the bolt sees 25*6=150 fp of torque.

So you can see the length of the wrench is vitally important element of the calculation.
 
lbs/ft.... to increase torque you can increase the force (lbs) or the moment arm (lbs). Doubling the moment arm will double the force. 1600lbs/ft... That's a 200lb man hanging off an 8ft bar, a 100lb man on a 16ft bar, or a 400lb man on a 4ft bar, 800lbs on 2ft, 1600lbs on one foot, or 1 lb on a 1600ft bar... all equals 1600lbs/ft. You can also use a gear drive to accomplish the same thing. 2:1 reduction will double force. I have also seen hydraulic torque wrenches that can put the HEAT to a large high torque fastener.

The cheapest way to do this is what they used to do in the Mazda dealerships to torque the flywheel nut on the rotary engines... they had a big long breaker bar with graduations, a set of bathroom scales and a chart on the wall. You went to the scale and weighed. You looked at the chart and it gave the position to place your hands on the breaker bar for proper torque. You got the nut tight as you could, then put the breaker on the nut horizontally, grabbed the bar with the designated mark between your hands and picked your feet up off the ground. Instant perfect torque.
 
For my money you CAN't put a long multiplier on to the handle of a clicker type torque wrench and expect it to change the torque.
It will still click at the same torque set on the thimble regardless of the length of the handle extension . A longer handle will just need less force to make it click.
The only way you can get a very high torque would be with a conventional and STRONG socket set, a very long extension handle,like a piece of scafold tube, and a strong spring balance so that you can determine the pounds applied at the tip of the handle, and the length of the handle from the centre of the square drive to the spring balance attachment point.
Torque = Feet x lbs if they are the units that you want. It matters not whether the handle is straight or not. It is just the straight line measurement that you need.

A 160 lb push on a 10 ft length of tube will do the job, but personally I think the socket will burst first or the square will twist like a piece of lead. I suppose it all depends on the size of the thread/ nut / square.

Davycrocket
 
Davycrocket - Agreed if we were doing a cheater-bar. But we are not using a cheater-bar in this exercise, we're actually extending from the nose of the wrench to the socket.
 
"For my money you CAN't put a long multiplier on to the handle of a clicker type torque wrench and expect it to change the torque."

Correct, but you can put the clicker type wrench on a longer handle and get any torque you want. An extension ARM, with a square drive on one end and a square female socket of exactly one foot will double anything you put on the handle (forcex2lbs/ft). Wringing off the drive or cracking the square socket or socket wrench are definite possibilities.
 
Mike C - I torqued like that using my son. I happen to know what he weighed at the time and he fit nicely on the wrench. I'm not sure I would have thought of the whole bathroom scale weight chart thing though. Somebody hopefully got a cost-saving award for that one. I bet you wouldn't see that in a BMW shop...
 
For short extensions, say 4" or less, of the type you are making you put the extension on 90 degrees to the beam of the wrench. This makes the torque error neglible for most purposes.

That bit of trivia doesn't help on your current problem. I would like to see pictures of a crowsfoot wrench that is good for 1600 lb.ft.
 
"For my money you CAN't put a long multiplier on to the handle of a clicker type torque wrench and expect it to change the torque."

Correct, but you can put the clicker type wrench on a longer handle and get any torque you want. An extension ARM, with a square drive on one end and a square female socket of exactly one foot will double anything you put on the handle (forcex2lbs/ft). Wringing off the drive or cracking the square socket or socket wrench are definite possibilities.

Knowing the length of the extension and the force applied to the end of the torque wrench is insufficient information to solve this scenario. You need to add either the length of the torque wrench or the torque that the wrench is set to trip at.

Henry
 
OK Thanks for that.
I was putting the cart before the horse.
Sorry to one and all .

Davycrocket
 
It's gotta be for huge fasteners, like 4 inches on the flats. I almost made such a tool once using a slugging wrench as the basis of the extension to get into a tight access situation for huge 1200FP (IIRC) fasteners. I calc'ed it would work, but I finally fixed the access problem with a modified socket instead of the slugging wrench - cheaper and this was a very cost-sensitive job. Been watching for a cheap hydraulic torque wrench to show up on ebay ever since...
 








 
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