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Pulley Versus Sheave

Posted by Crystal Carr on 3/15/2019 to Guides
Pulley Versus Sheave
Things just don't seem right with your beloved lawnmower: it's behaving a bit oddly and there's a strange sound emanating from the mower deck. You turn off the engine, and fall to your knees to investigate. After a few minutes of searching, you discover the source:

It's round, a v-belt runs through its grooved edges, and there's a hole in the center to attach it to the shaft of the motor... Your vast experience in this arena tells you it's a v-groove drive pulley. After removing the damaged product, you locate your lawnmower manual, and (with the use of a high-powered microscope) are able to decipher your Genuine OEM Part Number 
The description next to the part number says "Sheave, 7x3/4"

You see black around the edges of your eyesight... Now you have to question everything! Is the world truly spinning, or is it just you? How have you gone your whole life calling this a drive pulley, if it's really a sheave

Take a deep breath, you're not alone in this dilemma. This is, one could argue, the ULTIMATE QUESTION: 

What is the difference between a pulley and a sheave?

To fulfill our goal of answering this lofty question, I interviewed engineers who are experts in the field, I researched the etymology of the two words, I thought over my years of experience in this industries, and I read many articles and forum discussions on the subject. I am proud to say we will leave this blog with the definitive answer! 

Step 1: Check the dictionary

It's always a good idea to start off by seeing what the dictionary says, before going on ahead... 
Wikipedia defines a pulley as "a wheel on an axle or shaft...", and it defines sheave as "a pulley with a grooved wheel..."

Well, that suggests that all pulleys with grooved wheels are sheaves. Which would mean that the only pulleys we at Phoenix offer are unflanged flat idlers. That would be a surprising result, and call me biased, but I think there's more to this story. So let's go a little deeper and look at the origins of the two words.

Step 2: Etymology

The origins of words and languages is a fascinating study and can often provide deep and meaningful insight into the words we use. Let's take a look at what a Wiktionary search pulley-ed up:

PULLEY 
The word as we know it was first in common parlance in Middle English at some time between 1150 to 1500 AD. It was anglicized from the Old French(800-1400 AD) word, "polie." The French likely nabbed it from the Medieval Greek(400-1400 AD) word, polos, which means "pivot" or "axis". Prior to that, we can't be certain what they were called. Since this particular simple machine has been around since at least 200 BC, we can bet there is more to this story. 

SHEAVE 
Sheave, it turns out, can likely lay its origins at the feet of the proto-Germanic (~500 BC) word Scheibe, which means disk. Unfortunately, another definition of sheave is "bundle," so we can't be totally certain about this one. 
What this tells us is that the word "sheave" is probably a little older than "pulley." And, sadly, not much else. One interesting note is that the pronunciation of "sheave" is hotly contested as well. Depending on the region of the world, you may say "Shiv" or "sheeeve," with very different results. 

Okay, so maybe the history of words wasn't the right spin to take. 

Step 3: Ask the Experts

Next, let's look at the results of an impromptu survey of nine engineers and/or specialists in the field. Taken all together, they have a combined experience of more than one hundred years engineering and manufacturing pulleys and/or sheaves. The method of asking this question was carefully constructed: All persons surveyed were asked the exact same question in isolation, and were given ample time to elaborate on their explanation if they so chose. They were not provided a list of possible answers. While it cannot be guaranteed that no individual conferred with another prior to providing a result, effort was made to not sway the responses. The results were compelling.

Question: What is the difference between a pulley and a sheave?
The results were so compelling they are best presented in pie-graph form:

There you have it, my friends. 100 years of experience, one unified answer: Nothing. This was the first word all nine specialists stated. Phrased in a more complete manner, 100% of the engineers and specialist surveyed stated unequivocally that there is no difference between a pulley and a sheave.  


In all fairness, five engineers chose to provide additional detail, some at great length. Let's take a look at those results in a condensed format, because they are also quite interesting:

Pulleys and sheaves can refer to different things depending on the industry using the product - 2 responses
Most manufacturers of outdoor power equipment components refer to the products as pulleys. Manufacturers of components used in marine engineering will use sheave as a separate entity from pulley. Even with agreement on this aspect, the two were not able to explain how the terms would be used in separate industries.
 
"Sheave" is used exclusively to mean "drive pulley" - 2 responses
In effect, these two engineers have observed that in the population of their clientele, the word "sheave" is only used when a customer is talking about a pulley that has a hub, and is designed to connect to the shaft on an engine or motor. Both engineers prefer the term "drive pulley." This would certainly explain the slightly exaggerated scenario shown at the beginning, though!

"Sheave" is used instead of "pulley" in certain areas of the United States - 3 responses
All three engineers hail from the East Coast and have worked extensively with OEM Manufacturers from the midwest region of the United States. Many people in the midwest, both laypersons and engineers, will often prefer to say sheave instead of pulley.

"Sheave" is a heavier duty pulley - 1 response
This engineer's experience is that if a customer is referring to a "sheave" they are looking for a pulley that is either thicker steel or made of cast iron rather than a material such as die cast.

"Sheave" is never used in a close-loop pulley system - 1 response
A close-loop pulley system is mostly used in power transmission. If you are, for example, on the open seas and pulling a rope to hoist your sail up, the items providing you the mechanical advantage necessary to lift hundreds of pounds on your own are probably called "sheave" rather than "pulley".

Well, that was a whole lot of information! It turns out, that much like the difference between a snow thrower and a snowblower, while there may be a distinction between these two terms in some industries, because it inconsistently used, we can't glean any insights on what the product is without knowing a lot of background information. 

What this does mean is that if you call up 1-800-776-9315 and ask about a sheave instead of a pulley, we'll know what you're talking about and will be happy to help you find the best darned sheave for your needs! And to prove it, use code SHEAVE at check-out time to receive 10% off your order! 

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