My longstanding JAM Vise addiction is well documented here, and while I'm cognizant that I often turn this thread into a
show and tell I will yet again subject you to the latest arrival. It is something a little unique at least, hopefully of interest.
This 17.7kg (~40lbs.) chunk of the finest tool steel is the JAM HP150, although I paid a pittance for it, it sells for roughly $3000 and with a 120mm jaw width and 150mm span, is one of the largest units they make. But what makes it special is hidden inside the screw assembly, for this is a
Hydraulic vise. Before I detail exactly what that means, let's go back a few steps and talk about screws.
Screws are miraculous things, useful well beyond simply attaching things, precision screws are the basis on which most of the modern world has been built. They are fundamentally an endless inclined plane providing mechanical advantage. They have their trade offs though, and so special thread forms have evolved to try and compensate and optimize the screw form to its purpose. For a vise we are primarily concerned with how much pressure the thread can exert without failing and the amount of force required to turn the screw compared to the force it is exerting. The typical thread forms used in quality vises and clamps are Acme or Trapezoidal (metric), and in rarer cases Square or Buttressed thread forms might be employed. A sure sign of a cheap or lightweight vise or clamp is the use of conventional threads (although they certainly
do work, they will require more force for the same amount of clamping power and will fail under less pressure.) The main difference of these variants is to increase the cross sectional area increasing the
strength of the screw thread and to reduce the face angle in order to increase the efficiency.

If we look carefully at these threads we can see that the thread face angles are almost perpendicular with carefully relieved root areas. This is a modified square form, and it is one of the most difficult to produce with normal threading techniques like taps and dies (although since these are ground threads, it isn't a lot harder for them to make these over a more conventional thread form.) These square threads (actually they appear to me to be a 10° Modified Metric Square Thread) are the strongest and most efficient
(Machinery's Handbook 29th Ed., pg.1944) meaning that they are very good at converting the rotational torque into linear force, a higher thread angle is going to waste some of the force as the thrust vector perpendicular to the face is not as parallel to the screw axis. This efficiency becomes an important and non-intuitive factor because when we start clamping things,
thread friction suddenly becomes a really critical consideration!
A short time ago, an excellent YouTuber, Jason at
Fireball Tools did some
real world testing on different 'off the shelf' screw threads with the same pitch, but different diameters for use in a shop built vise project he was working on, and the results were quite interesting. I won't give the results of his experiments away, but it elucidates that there are a lot of surprising variables involved in selecting the right thread form, pitch, engagement length and diameter, especially if you want it operated by hand.
So how do we cheat? We can add some clever hydraulic advantage to our system! For those who may not be familiar, hydraulics use non-compressible liquids to convey force. Since pressure is force over area, one can use pistons with different areas (one with a small face, and one with a large one) to create something akin to a fancy lever that multiplies the force we are exerting. Nothing is for free, for this to work we are trading off displacement, the large amount we need to move the smaller piston, for the smaller movement at a higher force of the bigger one. But in this case we have a hybrid of both a screw and a hydraulic piston, so the very limited amount of movement of the larger piston is not really a problem. Let's look at how they are doing this.
Hidden inside the handle, jaw and the screw are an entirely self contained hydraulic mechanism. When the handle is rotated, the screw will close the moving jaw like a normal vise until it engages with the workpiece, then when we can no longer turn it due to friction, we can engage the hydraulic system with a gentile push of the handle. Once the fine internal threads of the handle start turning, they force the smaller 'long stroke' piston inside the handle to pressurize the system. This pressure transfers force to the larger area, slower moving piston inside the moving jaw. Due to the differences in area of the two pistons this force is greatly multiplied exerting some pretty astonishing clamping power (up to almost 15kN on this model) using only gentile hand power!
I'm sure there are conventional vises of similar size that can reach similar clamping strength with a long enough bar on the screw, but rotating threads under such pressures creates all sorts of problems and forces (including possibly messing with the vises position) and is much more likely to cause wear or damage. The hydraulic force in this vise is completely linear and the threads are not forced to slide when under such high pressures.
Hope this was worth the read!