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Monday, 25 January 2016 16:15

Spun vs. Split vs. Etc.

In this fourth and final installment we’ll be discussing the processes used to create pulleys. Spinning, splitting, stamping, molding, machining, casting and powder metallurgy each have their own strengths & weakness. We’ll also discuss best uses as some processes suit certain industries or applications better.

 

Spinning involves taking a stamped blank, typically having a domed or cup shape, and forming it between an upper & lower mold and form tooling. The part is held tightly between the molds (e.g. 25,000psi) and spun at high rates of speed (e.g. 900rpm). As the part spins, form tooling comes in and presses against it, forcing the material to change according to the shape of the form and mold which matches the desired groove profile. A typical pulley can use anywhere from two to four form tools, each attached to its own machine arm/holder which forms the part in consecutive stages.

Strengths: Pulley is a single piece which helps maintain consistent material thickness throughout the pulley

Weaknesses: Tolerances cannot generally be held as tight compared to processes like machining

Best Uses: Both drive and idler pulleys, standard single groove and poly-V pulleys, moderate to large sizes, all applications, steel

 


 …held tightly between the molds

 (e.g. 25,000psi)…


  

Splitting takes a flat stamped blank (think dinner plate) and forms it between a set of upper & lower molds and split/form tooling. The part is held tightly between the molds (e.g. 25,000psi) and spun at high rates of speed (e.g. 900rpm). As the part spins, the split tooling (think knife) comes in and presses against the edge of the blank. This causes the material to start to split in half. The form tool then comes in forcing the material to change according to the shape of the mold and desired groove profile. The material is pressed firmly against the molds, forcing it into its final shape. A typical pulley uses two individual form tools (one splitter/knife and one form), each attached to its own machine arm/holder which forms the part in consecutive stages.

Strengths: Pulley is a single piece which reduces thin points that can be created by assembling separate pieces.

Weaknesses: Tolerances cannot generally be held as closely as they otherwise can be in processes like machining.

Best Uses: Both drive and idler pulleys, standard single groove and poly-V pulleys, moderate to large sizes, all applications, steel

 

Stamping uses compound or progressive dies in a press to convert coils or sheets of metal into the desired shape/component. Depending on the size of the pulley a press uses anywhere from 30 to 800 tons of force.  The press may use what’s called a progressive die or a single stage (compound) die. A single stage die is just that, a die with a single stage where the pulley is stamped in one stroke/revolution of the press. A progressive die has several stages where the metal is moved from stage to stage and stamped into shape over the course of several strokes of the press. Generally, stamping in itself cannot create a whole pulley. It can only create one half at a time so two halves must be assembled using rivets and/or welding as a secondary process.

Strengths: Highly efficient and thus low cost

Weaknesses: Because of the size of presses and tooling needed to stamp metal, thinner materials are used which reduces overall strength compared to other processes like spinning and splitting.

Best Uses: Idler pulleys, standard single groove pulleys, small to moderate sizes, moderate applications, steel

 


 …press uses anywhere from

 30 to 800 tons of force.


 

Molding melts plastic pellets/beads by use of a reciprocating screw & heaters, forcing the now molten material through a small nozzle and into a mold. The material cools inside the mold, the mold separates in half and the molded part is removed. Molded parts from a cold runner system require finishing processes to remove excess plastic. This can be done by robotics. Hot runner systems do not necessary require extra processing. 

Strengths: Highly efficient process and thus low cost

Weaknesses: Material can melt or break/shatter under high load or high heat applications. Due to shrinkage during cooling, tolerances cannot be held as closely as with other processes. Inside and outside corners/edges must be rounded in order for the finished part to be properly removed from the mold.

Best Uses: Idler pulleys, standard single groove pulleys, moderate sizes, low load & low heat applications, nylon

 

Machining can take several forms including hobbing. In each instance a computer numerical control (CNC) or similar machine is used with precision tooling to remove material from a bar or slug. This process is controlled by a computer program entered into the machines terminal. Because each movement and process is controlled so finely, finished product can adhere to minute tolerances.

Strengths: The smallest of tolerances can be held.

Weaknesses: This is the slowest process by far and may require additional finishing processes depending on the part.

Best Uses: Both drive and idler pulleys, standard single groove and poly-V pulleys, timing pulleys, gears, small to large sizes, all applications, steel, aluminum, brass

 

Casting involves melting material and pouring that material into a mold. This can be done under pressure or not. Once the material cools the mold can be separated and the cast part (also called a casting) can be removed. The casting typically requires secondary operations including removing excess material and machining specified surfaces in order to hold tighter tolerances.

Strengths: Very strong and durable finished product.

Weaknesses: Castings are very heavy compared to pulleys made in other processes.

Best Uses: Both drive and idler pulleys, standard single groove pulleys, gears, small to large sizes, heavy load applications, aluminum, brass, iron (ductile & gray)

 

Powder Metallurgy is similar to both molding and casting in that a material is put into a mold to form the desired product. The difference is that a powder is used and, under pressure, forced into the mold. The subsequent part is fairly brittle and must be sintered in order for the powdered material to fully bond.

Strengths: More cost effective than many processes

Weaknesses: Tends to be more brittle than steel and other processes which reduces applicable uses

Best Uses: Bore adapters/plugs, hubs, small to moderate sizes, moderate applications

 


 Who knew there was

 so much to pulleys?


  

Throughout the course of these four installments we’ve covered quite a lot of information, from pulley basics & types to materials & processes used. Who knew there was so much to pulleys?

 

If you have any questions or just want to chat please don’t hesitate to send an email to This email address is being protected from spambots. You need JavaScript enabled to view it. or leave a comment here. We’ve been doing this since 1960. We know a thing or three.

 

New Hampshire Industries (NHI) 

68 Etna Rd, Lebanon, NH 03766

Last modified on Friday, 15 April 2016 15:41
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