My youngest son Luke never tires of my use of phrases from Star Wars. “Use the force Luke” is my favorite. I sometimes modify the phrases a little bit to suit the shop environment like, “Use the clamp force Luke.” or “Use the vise force Luke.” Luke doesn’t think a lot about forces generated by the vises in the shop. Do you ever wonder how much force is exerted by a screw operated vise on a work piece? My guess is that you, like my son, probably don’t. So that’s why we have engineers, to help answer all these inane little questions.
In your typical vise the screw acts as a force multiplier. A simple way to look at a screw is like a ramp wrapped around a shaft. The steeper the ramp is the quicker you get where you want to go but it is a harder climb. The other part of the force multiplication comes from the handle which is really just a lever. A longer lever means lower forces need to be applied but you have to turn it a much longer distance. As with most things in life tradeoffs need to be made. In the case of a vise the tradeoff is quickness of movement verses applied force to the handle to clamp your part. Ideally we want the vise to move quickly as we turn a small handle and apply lots of clamping force with only a small hand applied force to the handle. We know we can never get to this ideal vise so we try to balance things out to a level that is acceptable. So let’s look at how we can figure out the clamping force for a screw operated vise and see the tradeoff’s for a few example vises. As it turns out there is a very simple formula to let you figure out the vise clamping force.
CLAMPING FORCE = 6.3 X HANDLE FORCE X HANDLE RADIUS X SCREW TPI
The handle force is just the amount of force you apply to the handle with your hand. The handle radius is the distance from the center of the screw or hub to where you are applying the force and screw TPI is the number of threads per inch of the screw.
Let’s do an example of the Record 52-1/2 ED vise:
The Record has an 8” radius handle and a 4 TPI screw. Assume you apply 10 pounds of force to clamp a work piece, the clamping force works out to:
Clamping force = 6.3 X 10 X 8 X 4 = 2016 pounds of clamping force.
Most large wooden screws have a 2 TPI screw and they are desired for the quick movement when turned to clamp. But this faster movement comes at a price; higher force is required on the handle for clamping. To overcome this, wooden screws typically come with an extra-long handle of about 15”. Let’s look at a wooden screw example with a 3-1/2” hub, 15” handle and 2 TPI thread with the same 10 pounds applied to the handle:
The handle radius is actually 15 – (3.5 / 2) = 13.25 (you have to subtract half of the hub diameter to get the radius to the center of the screw.)
Clamping force = 6.3 X 10 X 13.25 X 2 = 1670 pounds of clamping force.
You can see that if you apply the same amount of force to the extra-long clamp handle you still come up short…..about 17 % less clamping force than the Record vise for the same amount of applied handle force.
Let’s look at one last example, a vise with 4 TPI screw and an 8” hand wheel. The handle radius in this case is going to be 4”. So for 10 pounds of force applied to the handle you get:
Clamping force = 6.3 X 10 X 4 X 4 = 1008 pounds of clamping force.
This is half the force you get with the Record vise. Or another way to look at it is that you would have to apply twice as much force to the hand wheel to get the same clamping as the Record vise.
You might be asking why I care about screw vises when I don’t even sell them. This little exercise points out the beauty of the quick action vise. You get quick action when moving the jaw up to the work and you get lower handle force with more clamping power. I also design my vises to mimic the action of vise screws so this formula is useful. I typically design my vises to work like a 4 TPI screw because that gives the best balance between low applied handle forces and good clamping force. May the vise force be with you……