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Everything You Need to Know about Pulleys

Pulleys are often referred to as blocks, and for the sake of this article we will consider the terms ‘pulley’ and ‘block’ to be synonymous.


How are they measured?

When someone refers to a pulley by physical size, most commonly they will be referring to the diameter of the sheave. The sheave is the wheel on the pulley. 


For example, a 3” pulley or a 75mm pulley refer to a pulley with a 3” (75mm) diameter sheave, as pictured. At Ronstan, we refer to size ranges as a “Series”. The series refers to the size in mm, so a Series 75 pulley will have a 75mm sheave. The pulley in the drawing is from our Series 75 Core Block range, part number RF74100.


HOW DO THEY WORK?

In their most basic form, a pulley gives the user a mechanical advantage to help lift, pull, drag, lower, or haul a load. Multiple pulleys can be used together to create a purchase system. Purchase systems can be as simple or complex as you’d like.

To the right we have a simple 3:1 Purchase System, which gives the user a 3x mechanical advantage. To haul a 30lb load, the user would need to exert 10lbs of effort.

What are they made of?

Pulleys can be made of all sorts of materials; traditional pulleys might utilise materials such as wood and bronze while more modern pulleys utilise high tech plastics and expensive metals. At Ronstan, all of our blocks are designed for rugged outdoor use and full exposure to the elements including salt water and sunlight. Our materials choices reflect these intentions - a few common materials found in our pulleys include:

Acetal: Acetal is a high strength polymer (plastic) used on many of our pulleys and sheaves. Acetal offers extremely high strength for its weight, excellent UV stability, and overall durability. This is one of the most common materials found on our blocks, and can be used for many different components including sheaves, cheeks, and even ball bearings.

Torlon®: Torlon® is another high tech polymer which is stronger and more expensive than Acetal. Torlon® is commonly used for ball bearings and needle bearings for pulleys intended for high strength and heavy loads.

Stainless Steel: Stainless Steel comes in many different grades, Ronstan often works with Grade 316 Stainless which is common for marine applications due to its high corrosion resistance. Stainless steel can be stronger, but also heavier, than high tech polymers like Acetal and Torlon®. It does not degrade in sunlight and in case of failure is more likely to deform rather than break entirely. Longevity, durability, and strength are characteristics of stainless steel, although it can oxidize if not properly cared for.

Anodised Aluminium: Anodised aluminium is strong, light, and very resistant to rust and corrosion thanks to the anodisation process. Its strength allows for clever weight saving designs in extremely high load blocks, creating the highest achievable strength to weight ratio. Ronstan’s Orbit Blocks™ are a great example of this.

How strong are they, and why?

A high quality pulley can boast extremely high working loads and impressive strength to weight ratios. The size, material, and design of a pulley all play important factors in determining the overall strength. Before a Ronstan pulley gets it’s published load rating, they each undergo a series of load tests which identify their efficiency and breaking strength.

While not directly tied to overall strength, efficiency plays a large part in a pulley’s overall quality and effectiveness. Efficiency is measure in friction, where energy lost creates heat instead of movement. It can be easy to be design an efficient pulley at low load or no load, but the challenge is to minimise friction as loads increase. A high quality pulley will perform well, with low friction, across the entire working load range.

With clever design and material spec, it’s possible to create pulleys that will handle loads many times that of their own weight. A shining example of this is the Ronstan RF35109HL, which has a 30mm sheave (only 1 3/16”) and weighs just 42 grams (1.5oz), yet has a maximum working load of 550kg (1,210 pounds). The breaking load on that same block is a whopping 1,650kg (3,360 pounds). To paint a picture, the RF35109HL pulley weighs about the same as a single golf ball and could sustain the load of the largest fully grown black bear ever recorded (which was estimated at 1,100 pounds).

Do they come in different configurations?

Of course, different applications and setups require different configurations. Some common configurations include:

A single block contains a single sheave.

A double block contains two sheaves, side by side.

A becket block has a tie-off point for the rope, known as a becket.

A fiddle block contains two sheaves aligned vertically. A fiddle block's sheaves are typically different sizes.