Pulleys and belt-pulley systems have an important place in machine technology. What are the types of pulleys and their areas of use? We’ll cover that in this article.
Types of pulleys by purpose of use
In machine technology, we can divide pulleys into three groups based on their purpose of use:
Power and Motion Pulleys
Power and motion pulleys are the pulleys we encounter almost everywhere, whether inside or outside the field of machine technology. That’s why, when someone says “pulley,” these are the first thing that comes to mind.
This type of pulley, with the help of tools such as belts, wire ropes, etc., transmits power and motion from one shaft to another.
Beyond the purpose above, power and motion pulleys are also made in different shapes so that they can accommodate the need to obtain different rotational speeds. Specifically:
- Flat pulleys,
- Stepped (graduated) pulleys,
- Conical pulleys.
Let’s get to know these one by one.
A — Flat pulleys:
If flat pulleys are attached to the shaft that carries them by elements such as keys, bolts, etc., they are called (Fixed pulleys) or (Keyed pulleys). Pulleys that are not attached to their shaft — that don’t rotate when the shaft rotates, or that can’t transmit motion to the shaft — are called (Idler pulley) or (Free pulley).
In an idler arrangement, these two types of pulley sit side by side.
For the idler pulley to run smoothly and to extend its service life, a bushing made of a bearing-suitable material is fitted into the hub, or alternatively two radial ball bearings can be placed there.
B — Stepped pulleys:
Stepped pulleys are essentially two or more flat pulleys of different diameters joined together.
Stepped pulleys are also called (Graduated pulleys) or (Cone-step pulleys).
Stepped pulleys are also called (Graduated pulleys) or (Cone-step pulleys).
The sum of the opposing diameters of two mutually driving stepped pulleys is equal and constant at every step (Figure: 14). That is:
D1 + D’1 = D2 + D’2 = D3 + D’3 = constant.
If this weren’t the case, when it became necessary to change the rotational speed of the driven pulley by shifting the belt along the endless belt-pulley axis, the belt would either not fit the next step or would be too loose for it.
C — Conical pulleys:
Pulleys whose rim resembles a truncated cone are called (Conical pulleys). The taper turning angle (A) of the pulley is (12°) or (15°).
To use conical pulleys in operation, we need to place two identical pulleys opposite each other.
Conical pulleys are used where stepless transmission of small amounts of power is desired. The belt is shifted along the pulleys in the direction of the arrows, changing the rotational speed of the driven pulley.
When the belt is at the middle of the pulleys, the rotational speed of the driven pulley equals the rotational speed of the driving pulley (ignoring belt slip).
By crossing the belt on conical pulleys, motion can also be obtained in the opposite direction. Since the pulleys are conical, the peripheral speed at every point is different. For the belt not to be damaged excessively, it needs to be narrow. (A belt width of at most 60 mm is recommended.)
Conical pulleys are used mostly in the paper industry.
GUIDE PULLEYS
When the axes of the shafts transmitting motion are not parallel, or are parallel but the power and motion pulleys are offset relative to each other, (Guide pulleys) are used to prevent the belt from slipping off. Because guide pulleys direct the belt, they are also called (Directing pulleys).
TENSIONER PULLEYS
These are pulleys used to reduce the belt’s slipping on the pulley by increasing the wrap angle
and to keep the belt continuously taut.
Tensioner pulleys are flat, idler-type pulleys.
Types of pulleys by construction and their areas of use
We can divide pulleys into two groups based on their construction:
I — Solid (one-piece) pulleys
II — Split pulleys
I — Solid (one-piece) pulleys:
Pulleys made from a single piece are called (Solid pulleys).
Because solid pulleys are easy to make, we encounter them in many places.
If mounting the pulley doesn’t present any particular difficulty and its diameter is small, it’s appropriate to make it as a solid piece.
II — Split pulleys:
If a pulley is formed by joining two pieces together, it’s called a (Split pulley).
Split pulleys are superior to solid pulleys in several respects. Specifically:
A — Split pulleys are easy to install and remove. (Suppose we want to mount a pulley somewhere on a transmission shaft, or remove the pulley that’s there. If the pulley is solid, the transmission shaft’s bearings, couplings, other pulleys, etc. would all have to be dismantled. We know how tiring and time-consuming that process is. Split pulleys, on the other hand, can be mounted or removed in a very short time without requiring any of that.)
B — Solid cast-iron pulleys with a large diameter (especially D > 1000 mm) and high peripheral speed (V > 32 m/s) can crack after casting due to residual stresses and centrifugal force. In split pulleys, however, the residual stresses after casting are negligibly small.
C — To cast very large solid pulleys, a foundry with a cupola furnace of suitable capacity and a crane setup is required.
D — Especially for large-diameter pulleys made in split form, their transport, installation, and removal are significantly easier than for solid ones.
To better understand split pulleys, let’s give a few examples according to the type of material used.
Split cast pulleys:
By “split cast pulley,” we mean pulleys that are cast from cast iron or aluminum and then machined afterward on work benches.
Cast pulleys are made in two pieces. The two halves of the pulley are joined together either through the spokes or between the spokes.
The joint through the spokes is the sturdier method, so it’s used more often.
Split cast pulleys can also be produced by casting them as a single piece and then cutting them in half afterward. (This is called “splitting” the pulley — the two halves of the pulley are cast joined together over a small area, and when this area is cut, the pulley separates into two pieces.)
The two large parts of the rim, and additionally the (D) part, which is the cast-iron section supporting the rim shown as (E), along with the dovetail female section, sit on the rim which is made of thick sheet steel. The two large parts of the rim are also joined together by (B) bolts.
Split steel pulleys:
The rim section of split steel pulleys is made of sheet steel. The belt’s sliding on the pulley generates heat. So that this heat doesn’t harm the belt and can be dissipated into the air, the thickness (Z) of the rim sheet is calculated using the following formulas.
MINIMUM = Z = D/300+2
MAXIMUM = Z = D/300+3
Important: Even though the unit of the pulley’s rim diameter (D) is millimeters, we must apply it in the formula.
Wood pulleys:
Wood pulleys are used for transmitting small amounts of power and are left outside the scope of (TS) standards. The rim section of the pulley is made by gluing together many segments so that their grain runs in opposing directions.
Wood pulleys are made in two pieces, and their attachment to the shaft is done with two or four bolts, without using a key. (When the bolts are tightened, there should be a gap between the two pieces of the pulley.)
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