Mechanical Advantage Principles

Let’s talk about what you really need to know as a heavy equipment mechanic. You’ve got to understand how that fuel’s hidden energy gets turned into real, usable power when it burns inside an engine. And then you’ve got to know how that power gets controlled and put to work in the machine. But here’s the thing—you also need to know how to use your own muscle and smarts efficiently when you’re out there servicing these beasts.

Now, engines are complicated pieces of work, with some pretty fancy principles and construction behind them. We’ll get to those later. For right now, I’ll tell you this much: an engine typically spins its crankshaft way too fast and with way too little twisting force—what we call torque—for you to use directly to drive or maneuver a machine. So what do we do? We step that speed down. We reduce it right in the machine, and in the same proportion, we increase the power. How? Through gears, levers, pulleys, hydraulic systems, and other methods. And it’s the same with your own strength—you build it up and apply it using wrenches, wedges, bars, jacks, and plenty of other tools.

Most of the time, when you’re getting an increase of force, you’re getting it through some form of leverage. And leverage has a bunch of definitions, but one of them is simply “increased force of action.” To really get how leverage works, we’ve got to go back to some basic physics. Don’t worry—it’s just for a couple of pages.

Mechanical Energy Principles :

Here’s what mechanical energy is. It’s what you put into that wrench when you’re trying to turn a stubborn nut. It’s what an engine delivers to its crankshaft. And energy also lives in a compressed spring, in a rock perched up high that might fall, and in anything that’s moving.

Now, here’s a fundamental rule: energy cannot be created or destroyed. It only changes from one form to another. The three forms that matter to us are force, distance or speed, and the heat of friction.

Force is a push or a pull. You measure it in grams, ounces, pounds, or tons. It’s the amount of pressure exerted by a source of energy, and it doesn’t matter how fast that source is moving, or even if the force is strong enough to move anything at all. You might be able to exert a hundred pounds of force just by closing your hand. That force might compress a spring in a scale, crush a container, or do absolutely nothing to a piece of metal—but it’s still a hundred pounds of force in each case. And when that force is twisting, we call it torque.

Distance refers to movement. It measures how far your hand moves when you pump a jack handle, and also how far the car moves up in response to that hand motion. Distance can also mean rotation around a fixed point, like the number of turns of a wrench as you tighten a nut, or the number of turns of a crankshaft or axle shaft to move a vehicle a certain distance or in a certain time.

We usually describe rotational speed in revolutions per minute—rpm for short. Rpm measures speed, but it also measures distance traveled in a one-minute period. To convert it to inches or feet, you’d need to know how far the point in question is from the center of revolution. But most of the time, we don’t bother with that. Rpm has become its own standard measurement.

And here’s a practical tip: when you’re figuring leverage, you can generally treat speed the same way you treat distance.

Mechanical Work:

Mechanical work is what you get when you exert yourself and actually move something. It’s the product of your exertion. In leverage discussions, we figure it with this formula:

Force times Distance equals Work.

Now, if you’re pulling on that wrench and the nut won’t budge, the distance is zero, so no work gets done. You might be sweating and straining, but in the physics sense, you’re not accomplishing any work.

And notice—this definition of work only cares about what gets done, not how long it takes.

Horsepower, on the other hand, measures both work and time.

Force Multipliers:

There are six so-called force multipliers that give you leverage. They are the lever, the wheel and axle, the pulley, the wedge, the inclined plane, and the classic sixth is the screw . Honestly, we might be better off calling them mechanisms or devices, because they don’t really match our modern idea of what a machine is. Plus, you can also get leverage through hydraulic and air pressure, through those far-from-simple hydraulic drives, and through a few other methods.

Shaft Configurations:

The power you deliver to a machine or device is the input power. Take a standard automobile transmission—power comes in through the rotation of the clutch shaft from the engine. That’s the input shaft. The connection to the driveshaft carries the power delivered by the transmission, and that’s the output shaft.

Usually, you’ve got one place for power input and one for output. But a truck or tractor often has two output shafts—one to move the vehicle and another, called a power takeoff, to run accessories. And if you’re looking at deep oil well drilling rigs, you might see several engines all driving one transmission, which means that transmission has an input shaft for each engine.

Mechanical Advantage Calculations:

Two basic factors are at play in most work, whether it’s done by machines or by people. Those are effort and resistance. Effort is the force applied by the person or the engine, without any reference to distance or speed. Resistance is the weight you’re lifting or the object you’re moving.

The ratio between the resistance a device can overcome and the effort you put into it is called the mechanical advantage—M.A. for short.

Here’s the simple formula:

Mechanical Advantage equals Resistance divided by Effort.

So if a man can just raise a 600-pound engine block with a 30-pound pull on a chain hoist, the mechanical advantage is 600 divided by 30, which gives you 20. That M.A. of 20 is a characteristic of that hoist, and it stays the same even if the load is lighter. For instance, a 5-pound pull would raise a 100-pound load.

Local Shop Note:

You know, I heard a great story from a mechanic over in Saugerties, New York, about a job that looked routine but turned into a real diagnostic challenge. I met him at an Albany, N.Y. equipment seminar. He had a chain hoist that wouldn’t lift a 600-pound engine block anymore. He checked it over and found worn gears inside the hoist. Replaced them, and that 20-to-1 mechanical advantage came right back—a 30-pound pull lifted the 600-pound block just like it was supposed to. Here’s what I took from that: mechanical advantage is only as good as the condition of the components delivering it. Worn parts rob you of that ratio every time.

But remember this—no machine can put out more work than you put into it. To get that 20-to-1 increase in lifting power, you’d have to pull that hand chain 20 times the distance the load rises, assuming there’s no friction to fight against.

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