Energy Basics
Objectives
In this lesson, we'll discuss the meaning and measurement of basic physics concepts that apply to hydraulic systems: energy, force, work, power, torque, and horsepower.
Energy
Energy isn't a physical thing you can touch. It's not like a photon or electron. It's just a term we use to describe the qualities of an object, just like the terms color or warmth.
When we say that an object has energy, we're talking about how much the object can change, or how it is changing.
There are many different kinds of energy.
- A fast moving object has a lot of kinetic energy.
- A hot object gives off thermal energy.
- Fuel is full of chemical energy.
- Electrons in motion have electrical energy and magnetic energy.
These different kinds of energy refer to how the object can change itself, or change other things around it.
Energy can be transferred between physical systems.
When energy is transferred from one system to another, some energy is lost in the process, because of friction.
For example: when chemical energy from diesel fuel is converted to kinetic energy in an engine, some of the energy is lost as heat and noise.
The heat and noise are lost energy because they aren't doing useful work.
The perfect engine would be silent and cool to the touch. But in the everyday world, no system is perfect. All the systems we see around us have some lost energy due to friction.
Thanks to friction, the useful output energy will always be less than the input energy.
But if you add up the output energy, plus the lost energy, it will always equal the input energy.
Input Energy = Output Energy + Lost Energy
The Law of Conservation of Energy says that energy can change from one form to another, but energy is never created or destroyed. Every last bit of energy always ends up somewhere.
The efficiency of a system is a measure of how much output energy you get compared to your input energy. The more efficient the system, the more energy is applied to useful work, and the less energy is lost.
One of the major advantages of hydraulic systems is the ability to transfer energy over long distances while minimizing the amount of energy lost.
Hydraulic systems can be very efficient, if they are well designed and well maintained.
Force
Let's start with an example.
A train is heading west, moving really fast.
The train suddenly crashes into a giant boulder! Whoa.
What's the boulder going to do?
- It's going to start moving west, because the train pushes it that direction.
- It's moving now, which means it received kinetic energy from the train.
What's the train going to do?
- It's going to slow down, because it transferred kinetic energy to the boulder.
- Everyone is fine, don't worry.
When they collided, the train exerted a force on the boulder. The train transferred energy to the boulder, and the boulder started moving.
So what exactly is a force? It's an interaction between two objects that can change how they move.
The most familiar example of a force is gravity. Your body's weight comes from gravity pulling you towards the center of the Earth. If you jump into the air, the force of gravity pulls you back down.
Multiple forces can act on an object all at the same time.
Here are some of the forces present in this hydraulic forklift, affecting the up-and-down motion of the load.
- Gravity is always pulling the load down, even when the load is moving up.
- Friction always slows the motion of the lift and the flow of its fluids, taking away their kinetic energy and turning it into heat and sound.
- Pressure is also a force. When hydraulic fluid or pneumatic gas is pressurized, it has a lot of energy stored up. When that pressure is released, the stored energy is transformed into the movement of flow, which pushes the lift up or gently lowers it down.
If an object stays still, or keeps moving at exactly the same speed, then all the forces acting on it are balanced. If an object speeds up, slows down, or changes direction, then there are unbalanced forces acting on it.
that does not mean
there is no force acting on it.
is countered by the pressure force
of the ground pushing back.
To figure out the strength of a force acting on an object, you need to know two things.
- Mass — how much matter is the object made out of?
- Acceleration — how quickly is the object speeding up or slowing down because of the force?
Let's say we drop a block with 2500 kg mass from an airplane. Earth's gravity tries to accelerate objects at 9.81 m/s2. How strongly is the force of gravity pulling on the block?
Force = Mass x Acceleration
Force = 2500 kg x 9.81 m/s2
Force = 24525 kg·m/s2
Force = 24525 newtons
Look out below!
The newton is the unit of measurement for the strength of a force — how fast the force could make a certain amount of mass speed up, slow down, or change direction.
- To make a heavy object change speed suddenly, you need a strong force.
- To make a light object change speed suddenly, you need a medium force.
- To make a heavy object change speed gradually, you need a medium force.
- To make a light object change speed gradually, you need a weak force.
Torque
Torque is how we measure the strength of a rotational force.
There are a few different ways to create a rotational force. The most common way is a linear force applied to one end of a lever arm (like a wrench or pliers), which turns an object at the other end of the lever arm (like a nut or bolt).
To calculate the torque in this example, multiply together the strength of the linear force and the length of the lever arm.
Torque = Force x Arm Length
Torque = 15 newtons x 0.2 meters
Torque = 3 newton-meters
Torque strength is measured in newton-meters — how many newtons of force, and how many meters away from the center of rotation the force is applied.
Imperial Units
In the imperial system of units, used in the United States and a few other places, the pound-foot is the unit for torque. Don't get confused: normally, the pound is the unit used to measure weight, but here it stands for pound-force. So the torque unit pound-foot is actually short for pound-force foot.
| Measurement | Metric Unit | Imperial Unit |
|---|---|---|
| Force: | newton | pound-force |
| Torque: | newton-meter | pound-foot |
Torque = Force x Length
Torque = 30 pounds-force x 0.5 feet
Torque = 15 pound-feet
Sometimes people say "pound-feet", other times people say "foot-pounds", or "pounds-feet" or "feet-pounds". These all mean the same thing. But sometimes, people just say "pounds", as in "900 pounds of torque" — that's bad because it's confusing. Stick to "pound-feet".
Work
When a force is applied to an object, and that object changes how it's moving, then the object gained or lost some energy. Work is the measurement of how much energy has been transferred to or from the object.
If a force is applied to an object, and that object does not change, then no energy was transferred, and no work is done.
To figure out the amount of work done when a force moves an object, you need to know two things.
- Force — the strength of the force performing the work.
- Distance — how far the force moves the object.
Let's lift a 15 kg mass block 4 meters into the air. To keep things simple, we'll say the amount of acceleration needed to overcome gravity and balance against the friction in this system is 10 m/s2.
Force = Mass x Acceleration
Force = 15kg x 10 m/s2
Force = 150 newtons
Work = Force x Distance
Work = 150 newtons x 4 meters
Work = 600 joules
Work is measured in joules. 1 Joule is the amount of work done when you apply 1 newton of force and it moves an object 1 meter.
The joule is the unit we use to measure work (the transfer of energy), but it's also the unit we use to measure the total amount of energy an object possesses. For example, the amount of energy in food or diesel fuel is sometimes measured in joules.
Power
Power is the amount of work done during a certain amount of time. It is measured in joules per second, also known as watts.
Earlier, we figured out that it takes 600 joules of energy to lift this 15 kg mass block 4 meters into the air. Now, let's figure out how much power we'd use if we wanted to lift it that high in 30 seconds.
Power = Work ÷ Time
Power = 600 joules ÷ 30 seconds
Power = 20 watts
If two identical objects are moved the same distance, but at different rates, the same amount of work is still being done to each.
Remember, this is because work does not take time into account.
But the amount of power being used will be very different!
Horsepower
Nowadays, the term "horsepower" has a few different meanings. The most common is "mechanical horsepower", where 1 hp equals 33,000 pound-feet per minute, or approximately 746 watts.
How did people decide on that meaning? Was there some scientific process? Nope. They just picked a number that seemed reasonable, and called it a day.
To figure out the horsepower needed to drive a hydraulic system, you need to know the system pressure and the volumetric flow rate of the pump.
Since this calculation is most commonly done in imperial units, that's what we'll use in our example here.
That constant, 1714, is similar to the conversion between watts and horsepower we looked at up above. There's nothing special or scientific about this number, it just happens to be the value that was picked for the meaning of horsepower.
Recap
This lesson introduced some basic, but important, physics concepts that relate to hydraulics.
Energy
- Energy is how much a thing or object can change, or how much it is changing.
- There are many types of energy: kinetic, chemical, nuclear, and many more.
- Energy can be transferred, but it is never created or destroyed.
- Efficient systems lose very little energy to friction.
- Energy is measured in joules.
Force
- Force is an interaction between objects that causes one or both of them to speed up, slow down, or change direction.
- Forces can be linear or rotational.
- Torque is the term we use for a rotational force.
- Linear force is measured in newtons, and torque is measured in newton-meters.
- Multiple forces can act on the same object all at once. Gravity, friction, pressure, and kinetic forces are always present in hydraulic systems.
- If an object isn't moving, then all the forces on it are balanced.
And the rest...
- Work is the total amount of energy transferred when a force moves an object, measured in joules.
- Power measures how quickly that energy is being transferred, in watts.
- Finally, horsepower is an old term that means the same thing as power.