Force, Displacement & Bloody Knuckles
Is there anything more annoying (painful and frustrating) than busting a knuckle when working on things in the shop? To many of us, blood sacrifices are just part of getting a project done, but it doesn’t have to be that way. This post is NOT about iron gloves or other bloody knuckle avoidance techniques, but rather a paradigm shift to understanding why. Here’s a quick perspective on Applied Force and it’s relationship with desired movement. In this case, for tools in the shop, and avoiding the “Bloody Knuckles” paradigm.
3 Force Concepts:
Varying degrees of force are all around when using tools in the shop, and they are usually applied in one of 3 different formats. Understanding the way forces are applied and the resulting reaction when the task is achieved (or when something fails) can give a perspective on which applied force is best. First, the 3 formats:
- Force Irrespective of Displacement;
- Displacement Regardless of Force;
- Impact Loading.
These are not absolutes in any way, but we’ll use the concepts here for perspective.
Applied Force Without Regard To Displacement
Applying a force in a way that does not depend on displacement (with in reason) is the fundamental way our muscles work. As an example, when you put a wrench on a tight bolt with the intent to loosen it, you apply a force to the wrench and if the bolt does not move, the force appears to do nothing. On the other hand, if the bolt head suddenly breaks off, the wrench (and our hand) instantly begin to move, sometimes just enough to bust a knuckle.
Our ability to “un-apply” the force is a function of feedback — when we feel a sudden movement, then our brains tell the muscles to quit applying the force.
This type of application is also present in other areas of the shop like pneumatic tools. Pressure applied via compressed gas has an expansive element that won’t stop immediately when resistance is released. To illustrate, think of a balloon. When the balloon pops, the forces continue to projectile the fragments as forces dissipate.
Movement Without Regard To Applied Force
This second application type can best be illustrated by thinking of a hydraulic jack. Within the limits of the jack capability, when you pump the handle on the hydraulic ram, there is a displacement. If there is nothing on the ram (nothing to lift), it will move roughly the same distance as when a high force is there (like lifting a car). This distance really doesn’t care if it’s lifting your car or lifting a feather. When you pump the handle, the ram moves. (Of course, there are limitations, and this paradigm is not absolute. Just note the perspective.)
When applying movement without regard to force, things only move the specified distance regardless of the need force. Another good example is that of sheering material in a shop press. The pieces are in place, the ram moves into contact, then displacement of the ram makes a high force. When the piece suddenly breaks, there is no more force on the ram, yet the press moves the same distance with each pump stroke. And, the press ram doesn’t suddenly fly across the room when the force of shearing material suddenly completes. (Yes, there is some rebound from energy stored as deflection of the shop press frame, but in general, those movements are small compared to the applied force.) Imagine doing the same task with compressed air — then the ram would project like a large bullet till it found something sufficient to stop it.
The third type is impact loading. The perfect illustration is a hammer and a nail. How much press force does it take to push a nail into a board? Can you push a 16-penny nail into a 2×4 with your hands? Impact loading — the action of hitting the nail with a moving hammer — applies some really interesting loads. With a relatively small hammer one can easily drive the 16-penny nail into the 2×4 with just a few strikes. It can also bend that nail in ridiculous ways when the strike is off just a bit.
Truly, impact loading is amazing as it makes things operate in a little different manner than normal. Think about the stock of straw that penetrates the barn wall in a tornado. There is no way to push a piece of straw through the wall — only with the impact of amazing speed.
Applying The Right Forces
So there’s the perspective. Being aware of the different types of applied force can go a long way to choosing the right methods for each task. Think about it next time you’re using a wrench in an area where there is a risk of bloody knuckles.