Gripper Calibration Tool: The Torquer

[sigmfsk]

I wanted to measure the closing torque required on my grippers and thought of an easy way to measure them. A while back, I read about PDAs calibration of grippers and how it was difficult to measure in a machine due to one handle lengthening on closing, and how the handles didn't close in a parallel fashion, etc.

My wife and I are on vacation, and we got chatting about the problem, and I

made a simple device that consists of basically two long lever arms connected by the gripper to be measured. The device, which I'll call the torquer, is closed by holding vertically at chest level, like a large set of garden shears. One lever arm is a hollow pipe. The other lever arm consists of a gripper handle receptacle mounted to two opposing torque wrenches which show the torque reading.

The torquer's readings correspond nicely with the torque numbers which I calculated for our "vacation" grippers (trainer, 262, Lemley, Elite) based on the weight required to close them, as measured 3 3/8" from the axis of the spring body. (the torquer is double duty in that the torque-wrench side can be used to measure closing torque with a torque wrench, or measure closing weight specifically at the 3 3/8" distance). The 3 3/8" distance was chosen as a guesstimate of where the average force of the hand would be applied to the gripper handle, and because it didn't really matter as the numbers could be converted to any point on the handle, and because it was the standard computer distance, one attoparsec, from the end of the gripper handle.

I'll attach some pictures of the torquer upon return from vacation in a few days, but my surprise is that both the torquer and the postage scale have readings for each gripper higher than expected.

weight required to close, as measured 3 3/8" from axis of spring body, and calculated inch-pounds to close

gripper pounds inch-pounds

Trainer: 61 206

262: 133 449

Lemley: 154 520

Elite: 185 624

These numbers seem just way too high compared to PDAs measurements. PDA has a measured range of 120 to 154 inch pounds for Trainers, and our trainer is on the weak side. I think the "weight" numbers make it easier to discuss what's going on, as it eliminates possible error in the design or operation of the torquer.

Has anybody else calibrated their grippers in any fashion? PDA's site shows that the torque rating by the Spring Manufacturers Institute of the Trainer's .225" A228 music wire is 173 inch-pounds. And with a gripper, we're closing the spring farther than designed, so I would expect the closing torque reading to be higher than the rated 173 inch-pounds.

The plot thickens...

[mooinabc1]

The best way to pounds is using real weight to

the poundage on the gripper.

Just make tool that can be added weights and

that will give the real numbers.

[sigmfsk]

The 61 pounds for the Trainer is an actual weight, with no torquer device, just a reading on a scale. All the numbers in the original post are real world weights, not readings on the torquer. The torquer makes it more convenient to make measurements, especially of really tough grippers, but I recognize that its easier to discuss the torque calculations without the device.

It took 61 pounds, as measured by a postage scale, applied to one gripper handle of my Trainer, exactly 3 3/8" from the axis of the spring body, for that gripper handle to just touch the other solidly mounted gripper handle.

It seems that it should be simple. 61 pounds * 3.375 inches = 206 inch-pounds.

[Clay Edgin]

sigmfsk, vacations are not for work!

The pound ratings on the scale are a good way of comparing your grippers and the numbers you came up with sound like the numbers we had when Gorilla Hands, Porkbone and I got together last December and did something similar with the scale. Interesting to note that the toughest #4 tested closed at about 255lbs, compared to the easiest which was just under 200 I think.

[Lyle]

clay I wasw gonna do some thing similar with my grippers, what numbers did you get for #3's?

[sigmfsk]

Thanks for the info Clay! Now I don't think I'm losing my mind, and I think you hit upon a good measuring standard. I got my grippers, measured them by putting my palm against them and closing against a scale. I compared those numbers against the exact numbers measured at the closing point 3 3/8" from the axis of the spring body. For me, at least, my palm against the gripper handle against the scale puts the force 3 1/8" from the axis of the spring body.

I'm guessing that most people pushing a gripper against the scale would put their palm in about the same position and press in about the same way. So I propose the standard:

Pounds required to get the handles to touch when pushing against a scale with the dog-leg against the scale (with the force presumably located at a point 3 1/8" from the axis of the spring body).

Here's what I get for some grippers, in pounds.

Trainer__65

1______104

243____109

2______137

262____146

Lemley_161 (unseasoned)

280____166

GM____188 (unseasoned)

3______195

E______200 (unseasoned)

The above numbers are with the dog-leg against the palm (or against a non-moving lever arm mounted in a power rack). I did verify PDAs finding that it took slightly more force to close with the dog-leg being the moving lever arm.

Its so easy to measure the closing force (especially of a trainer) against a scale, and the numbers are so much higher than PDAS numbers, that their closing inch-pound numbers seem suspect. I'll shoot them an e-mail and see if I find anything.

It it seems that my 3 might be on the tough side, using your 4 measurements for comparison. Then again, your hands might be tougher and be able to put the weight against the scale further down the handle.

At least the torquer is a fun tool for comparing grippers within our little grip club. I placed a pic of it in the gallery.

thanks,

arthur

[3djim]

Re: measuring forces

The actual process of pushing a gripper on a scale with palm is vague and doesn't appear to offer close accuracy for guessing force-to-close. I've tried this for a Trainer and got FTC numbers all over the map depending on where I place the gripper on the scale (actually, on a piece of plywood on the scale to better distribute the applied force) and how/where my hand pad contacts the moving leg. There just appears to be too many variables (especially, how the pushing hand contacts the leg ... eyeballing just is too arbitrary) to trust the results. So, I guess I don't understand exactly how you focus your power, on the moving leg, at a specific lever distance from coil axis.

I suppose you could devise a "pushing platform" that molds to your hand, yet only contacts the gripper leg at one "position". Or, as some have suggested, forget the scale and just hang weights off the moving leg. Some creativity is necessary to make the measuring contraption stable and safe to use.

I've thought about a simle setup to do just that (I haven't constructed it yet): mount the fixed leg in a slightly hollowed out groove, on a horizonal post where a couple heavy duty nails are there to steady the vertical position of the coil. Then strap down the leg with a couple band clamps. Then hang they weights at a chosen "point" on the moving leg using a loop cable. fixing the contact of the cable to the leg should be done so that there will be no slippage as weights are added.

Also, I believe IronMind grippers have targeted their estimates of force-to-close to the handle mid "point", which usually is ~2 5/8" from axis. This is nearly equivalent to a one finger, rather than four, approximation. You can see the "rings" on their gripper legs where they apply that canonical force. Thus, an IM gripper rated at 100# is one that apparently is targeted to FTC by applying a single force at the ring position. Of course, their inherent manufacturing errors are other issues regarding how close to the target FTC they expect to get, statistically.

Re: measuring torque

I like your apparatus, but believe it needs refining to get more accurate results. From the picture, it appears the torque value will always be lower than it shoud since the torque reading is apparently not taken when the handles actually touch ... that is, it looks like that reading is taken when the handle enclosures touch. anyway, I personally believe that the torque technique is the way to go since it can be used to get forces at any "point" along the leg, as you mentioned. However, it can also be used together with the angle through which the handle moves. Torque is linearly proportional to that angle, so that for a real tough gripper, you only need to move the handle a small amount to get the proportionality constant. Then just apply it to the full angle.

So, your investigation and Qs mean that there are those in this group who want to better understand quantification of gripper forces-to-close based on geometry and material properties of the grippers.

You might search for similar posts about this topic. It pervades the group archives. There are widely varying opinions, with most gripsters favoring a simple "squeeze and compare" approach since there are so many variables. That is a good practical approach as long as the grippers used for testing purposes are consistent in FTC. Thus, we can understand the reason for the Mash Monster series.

[sigmfsk]

Thanks for your comments. The piece of pipe that receives the torque wrenches receives them at 3 3/8" from the axis of the spring body (by design). I used this piece, without the torque wrenches, to exactly determine how much weight was needed, exactly at the 3 3/8" point to close a variety of grippers. Just hang weight from that point until the handles just touch. Or just slide a long rod through and push against the rod against the gripper on a postal scale. Similar readings were obtained.

I then verified that the device with the wrenches accurately measured the closing torque. A conversion is needed. Because if the torque wrench shows 120 pounds, that reading is over 12 inches (the length of the needle on the torque wrench), indicating that 10 pounds is operating over 12 inches. The mounting point is 3 3/8" from the axis of the spring body, so its actually 10 pounds over 15 3/8".

As a note, the torque is measured when the handles just touch. There was sufficient pipe removed on both lever arms so that the handles can touch without interference.

And theres another slight conversion in that the 10 pounds isn't applied at a 90 degree angle to the gripper handle. Upon closing, the force is supplied slightly different than 90 degrees, so the vector has to be considered. But in the end, everything measured what was expected based on the original weight measurements.

It was fun to build, and the grip groupies love it!

[3djim]

sigmfsk---

You mentioned that you were going to contact PDA to better understand their torque ratings. What did they say?

I also get much higher torque values than they do. But, my torque values are consistent with IronMind's relative to IM's FTC targets (I realize these targets are only statistically achieved, given the many variable manufacturing errors). E.g. the IM target for a Trainer is 100 lbs FTC where a hypothetical force is applied at a "point" half way down the handle. i.e. ~2.7" from coil center axis. Hence, for a Trainer, the IM targeted torque is 270 inch-pounds. At 3.375" from the coil center, FTC woud be 80 lbs. And, at handle end (~4.5" from axis), FTC would be 60lbs. This latter figure is very roughly what I am getting from scale readings (range 60-70, depending on how much of my pressing palm contacts the handle near its end).

[sigmfsk]

Well, (sheepishly), I haven't e-mailed them yet about the torque. I'm waiting on a custom Gripanator handle from them (a smooth handle against the fingers vs. the knurled handle that continues to tear all the skin off my hands [and tear through gloves]), and was waiting until that issue was closed before before asking about the new torque issue.

But its just a simple question - I'll e-mail right now and keep you posted.

[menace3000]

I was wondering how come the ratings of the grippers are so different from the other ratings...that is elite is rated no higher than 200lb when it's rated around 325lb in other places....tks..