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Findings on Clamps

Somehow I got obsessed with clamps today. The other day I saw a report on TV about pain assessment with a so called Algometer. There a special press is used to compress a finger until the patient says it is starting to feel painful. This is called the “pressure pain threshold test”. Since the area of the press and its pressure is known a numerical value can be given for this pain threshold. Same goes for temperature. I found a report where they tested at which temperature a hotplate is so painful, the the test subject needs to get their hands of. They found 47 °C for woman and 50 °C for man. Maybe I will access this topic later in the context of candles and wax play.

Back to the clamps. Through googling I found that they also use standardized clothes pegs on the earlobe for this pressure pain threshold test. Does testing clamps on the earlobe sound as familiar to you as for me? All right. So their test clothes peg did have an clamping force of 10 N and an area of 100 mm², hence a pressure of 0.1 MPa on the tissue of the earlobe. In our circles clothes pegs are not known for their excruciating pain. So, how about other clamps? REAL clamps used in BDSM.



Well it turns out the measuring the clamping force is far from being easy. With a little training, some fantasy, a scale for measuring baggage and a bench vice it is possible to get a rough comparison. This is far from being scientific correct and as a reviewer of a peer-review-magazine I would definitely reject such a crude paper. But in this case, so what.

I put the clamp into the vice and measured the force that is require to open the clamp. With a caliper and some geometry I extracted the force the clamp will exercise on its clamping surface. Since all investigated clamps are spring loaded the force will be linear with the opening angle of the clamp. Usually one should choose a certain sample thickness. Lets say 5 mm for a squeezed nipple. In practice I neglected this and tried to be around this value, since it already was hard enough to make sure the hook of the scale was not slipping of.

The area was measured with a caliper and either taken as rectangle or circle. I found it very problematic to find an area value for those nasty toothed clamps. Area may be of an order of magnitude or more for those. This will have a great impact on the pressure on the tissue.

Definition: MPa is a unit of pressure and defined as 1 N (roughly equals a weight of 100 g) per 1 mm²

Baseline: Rough comparison, not more.


Because of the problem of measuring the area of the toothed clamps I divided them in two groups. One toothed, and one un-toothed. The results are sorted from light to intense. The measured order and the subjective order tested on my earlobes fit. Will be verified on nipple and labia, once the test subject is back from vacation.

Un-toothed Clamps

  1. Even weaker as the clothes peg under investigation: A BDSM-Clamp. This clamp is based on a crocodile clip but covered with a plastic protector. Originally the clamps even had a thumb screw to lessen the pressure. Clamping force: 6 N, Pressure: 0.1 MPakinklab_bdsmclamp
  2. My version of a clothes peg managed to be more intense than the standardized one for the pressure pain threshold test. Clamping force: 5.6 N, Pressure: 0.16 MPakinklab_clothespeg
  3. The much-loved Clover Clamp (or Butterfly-, or Japanese-Clamp) made – to my surprise – only a quite low clamping force. Slightly more than a clothes peg. But, this clamp will get a considerable tighter grip once pulled due to the internal mechanic. Nevertheless, without pull or weight: Clamping force: 6 N, Pressure: 0.22 MPa [Update] Examining the clamps internal lever mechanic I calculated 1.4 N additional clamping force per 1 N pull on the chain. This raises the pressure also by 0.05 MPa per 1 N force applied to the clamp’s chain.kinklab_cloverclamp
  4. A clamp I found at a hardware store used for holding things together while gluing. Clamping force: 18 N, Pressure: 0.6 MPakinklab_hardware
  5. A so called Kelvin Clip used for 4-point probing in electronics. Sold for extortionate prices at BDSM stores as nipple electrodes, or at every electronic distributor for next to nothing. Turned out to be surprisingly strong. Clamping force: 10 N, Pressure: 0.8 MPakinklab_kelvinclip

Toothed Clamps

  1.  To begin with a black plastic covered 2 mm crocodile clip for electronics. I use it primarily as an electrode. Clamping force: 5 N, Pressure: >0.3 MPa. Subjective this one is more painful. I would rate it as equal to 0.7 MPa judging from the un-toothed clamps.kinklab_blackcrocodile
  2. This is a goody for pony play with a bell on it. Also toothed and has quite a bite. Clamping force: 3.3 N, Pressure: >3.3 MPakinklab_bellclamp
  3. A small crocodile clip also from electronics. I attached some strings to it to spread the labia quite nicely. This is sometimes referred to as spider. Clamping force: 6 N, Pressure: >4 MPakinklab_smallcroco
  4. The medium crocodile clip that directly attaches to 4 mm plugs. Clamping force: 12 N, Pressure: >6 MPakinklab_mediumcroco
  5. And my favorite little bugger. A clamp used for attaching curtains to a track. Small but very ouch. Clamping force: 12.5 N, Pressure: >14 MPakinklab_curtainclamp


By the way. The average Algometer goes to pressures up to 4 MPa. For medical assessments this seems enough to make a finger tip hurt enough. On the other hand, a clamp with 4 MPa attached to a nipple or the labia will definitively hurt quite strong.

How do pressure in MPa and perceived pain correlate. I assume that as almost always in nature the two go logarithmic. The hardest clamp hurts a lot, but not by factor 100 compared to the clothes peg. So what is learned from this? I dunno. But should I decide to engineer my own clamp, I now have a starting point.

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