# How can I measure the strength of a wrist gripper?

I've been given a hand gripper and I want to test its strength in $$kg$$. Of course, I could compare it to grippers with known strengths or grip it to the maximum and apply the same force on a bathroom scale.

However, is there a more objective test? Accuracy is nice but not critical.

Edit:

Hanging loads on the gripper is impractical as the strength of a gripper can be as high as 200$$kg$$.

Edit 2:

As mass of 12$$kg$$ pulls the gripper 35% of its full way - is it correct to say that, as the grip movement is in the elastic range, its full load is around 35$$kg$$?

Don't think this is possible accurately without comprehensive tools or an extensive procedure. Also, $$kg$$ is not a unit of strength nor force and is not applicable in this situation. You are thinking of weight.

One crude way I can think of is using objects of known mass, such as water, and hanging them on to the gripper. When a particular mass pushes the gripper all the way, this will give you an estimate. Water might work well since $$1\space L$$ is $$1\space kg$$ and you can keep changing it to match the strength.

You will always need something to compare to (such as mass) no matter the method, since the definition of a $$kg$$ is not intrinsic. Can't find a $$kg$$ in nature. You could use springs but you'll need a spring constant value which you can find ($$F=-k\Delta x=mg$$).

• Aren’t kilgrammes a unit of mass? – user207455 Jul 16 '19 at 21:25
• @SolarMike Yes and mass is only about the amount of matter in an object. Weight, however, is the force due to gravity and mass. So when you're measuring it you're actually finding the force required using weight which is dependent on mass ($F=ma$), – KingMongolian Jul 16 '19 at 21:28
• Op said kgs and you said the op was thinking of weight in your first paragraph... – user207455 Jul 16 '19 at 21:29
• @SolarMike OP talked about measuring strength and force in kg, and mass is not strength or force. Weight is a force. – KingMongolian Jul 16 '19 at 21:32
• All the more serious grippers come a strength denoted in kgs. That's why I asked about a kgs measurement. I thought a bout the weight test that oyu offered but it seemed to me not so practical. – OMGsh Jul 16 '19 at 21:32

You can't expect a precise measurement for an imprecise concept, but here's an approach that might sort of work.

Holding the gripper, mark a point between your middle fingers for one side, and somewhere that your palm touches the other side of the gripper for the other side. Since you grip at an angle, you get different leverage at different parts of the handle and you need to pick one part of the handle to make your measurement.

While holding the gripper closed, put some rubber bands at the marked spot. You can't get them all precisely there, but just do the best you can.

Release the gripper and measure how far your rubber bands let it open. Do this repeatedly.

Remove the rubber bands and make something else to stretch them that you can measure, like suspend a bucket from them and measure how much water it takes to stretch them the same distance. Try to get the handles the rubber bands are attached to be the same diameter as the ones on the gripper. Do this measurement repeatedly.

Check whether the rubber bands get significantly weaker with repeated stretching, by looking for doing a least-squares on the two sequences of measurements, and see whether the slope is significantly different from zero. If it is, and if you want some sort of precise value, then you should repeat the trials enough to get your slope estimate as precise as you want it, and then correct for that.

If your rubber bands are too weak, then get stronger rubber bands. Varying thicknesses of some sort of inner tube, maybe.

It sounds like your gripper has a way to adjust the resistance over a wide range. Unless it clicks across quantum adjustments, likely adjusting the thing will provide big errors. It might be hard to adjust it the same way twice, to the point that you needn't care about precision better than that.

• This is a nice direction. I hoped for more "physics" solution but maybe there isn't. Btw maybe my phrasing was unclear - my gripper has a fixed strength. – OMGsh Jul 17 '19 at 22:58