Pulleys and counterweights have been used for centuries to gain mechanical advantages in war machines and industrial equipment. The physics behind them is fairly simple, but their importance is paramount.
Be the first to read our latest stories, analysis, and trend-spotting on tech from every corner on earth. Sign up for our weekly newsletter today.
Stay on top of the latest engineering news just enter your email and we’ll take care of the rest.
Stay ahead with the latest science, technology and innovation news for free
By subscribing, you agree to ourTerms of Use andPrivacy Policy. You may unsubscribe at any time.
In our modern world, pulleys and counterweights are used in a wide variety of applications. For example, every time an elevator moves, a system of pulleys and counterweights are engaged. In terms of machine design, pulleys are perhaps the easiest way to gain a mechanical advantage. Another common way to increase your output force in modern mechanics would be levers, which pulleys function very similar to.
Without getting into all of the various pulley set-ups, at their core, pulleys have a wheel and a rope. A pulley with only one fixed wheel simply reverses the direction of the input force relative to the mass being pulled. With only one wheel, in order to lift a mass of 100 kg (220 lb), you would need to exert a 100 kg force equivalent (1000 Newtons) on the other end of the rope.
When more wheels, or blocks, are added into the simple machine that is a pulley system, you gain more and more mechanical advantage. With a system of 2 wheels, you could lift a 100 kg weight by only exerting 50 kg (110 lb) of force equivalent to the rope.
Calculating input forces gets a little more complicated the more blocks you add as well as through different set-up variations. However, even the most complex pulley systems can be understood by adding up lengths of the different stretches of rope and generating equations from derivations.
RELATED: 19 MECHANICAL ENGINEERING INNOVATIONS THAT HELPED DEFINE MECHANICS TODAY
The most important thing to remember is that pulleys aren’t magical systems that simply transform a small force into a larger force. This may seem true if you focus on the input and output, but the mass of what is being lifted is always translated into the fixed points where the pulleys are attached to your rigid system. All of this can be calculated through some simple physics.
Rather than transforming forces, we can say that a system of pulleys and ropes manages forces to maximize how much mass you can lift or move.
Now that we have a basic grasp of pulley systems, we can dive into counterweight systems to gain even more mechanical advantage in machine design. Counterweights are both used in pulley systems, like elevators, as well as lever systems, like a crane. In terms of crane counterweights, it is all about moments.
To keep a crane arm rigid while lifting a 100 kg mass that is 10 m (32 ft) away from the fulcrum, you would need a 1000 kg (2200 lb) counterweight 1 m (3.2 ft) from the fulcrum on the other side to stabilize the crane arm. In cranes, counterweights typically maintain a constant mass, so in order to adjust for different loads, the counterweight usually has the ability to be moved to adjust the moment. This is obviously a very rudimentary explanation of crane counterweights. However, this is a great example of how pulleys create a mechanical advantage in the systems in which they are employed.
RELATED: ENGINEER DISCOVERS DAVINCI'S HIDDEN KNOWLEDGE OF FRICTION
In terms of pulley systems, a counterweight simply helps to apply an input force to lift an object. The easiest way to demonstrate this would be to look at an elevator system. In typical elevators, there is a counterweight attached to the other end of the pulley system equivalent to about the weight of the car at 50% loading. Counterweights don’t do all of the work in modern elevators, but they help stabilize the system and reduce the load on the elevator motor.
See below for several different examples of pulleys to see how they impact a given weight.
Assuming an elevator car was loaded at 25% of its capacity, all the motor would need to do is brake the elevator car on the right floor (when going up). The counterweight in an elevator also means that if a motor brake were to fail, the car would be slowed from falling quite as fast as it would otherwise. Elevator systems as a whole are a little more complicated than explained here, but counterweights are the essence of their function.
Specifically, Destin from the YouTube channel Smarter Every Day discusses a type of pulley, referred to as snatch blocks. This type of pulley allows for cordage to be threaded through them with ease, as they split apart in the middle. This saves the user from having to thread the rope or chain through the pulley mechanism on the onset of use. In this way, snatch blocks are a great way to leverage the physics of pulleys and their mechanical advantage on the fly.
For full access to all features and product updates.
Already have an account? Log in