Flexible connector drives are simple devices commonly used to transmit torques and rotational motions from one shaft to another. Power is transmitted by a flexible connector (belt) placed on pulleys that are mounted on these shafts to reduce peripheral forces. The transmission ratios of torques and speed at the driving and driven pulleys are ascertained by the ratio of pulley diameters. A system of belt pulley drive, drawn using AutoCAD solid models, is shown in Fig. 10.1. The figure clearly explains how the belt transmits motion from the driving shaft (may be attached to an electric motor or any prime mover) to the driven one. The belt is referred to as a non-positive flexible connector as it communicates motion by means of friction between the belt and the pulley surface. The amount of transmission torque depends on the friction coefficient of the belt and the pulleys as well as on the contact pressure between them.
FIG. 10.1 Belt Pulley Drive
A belt pulley drive is very easy to install and requires little maintenance. It is relatively reliable and easy to handle. However, power transmission capacity reaches its limit when the belt starts to slip. In order to increase the torque transmission capacity, a wedge-shaped belt (V-belt) is frequently used, the details of which will be explained later in this chapter.
The major parts of a pulley are shown in Fig. 10.2. Pulleys may be made of cast iron, wrought iron, steel, wood, or even plastic, depending on the size and working conditions. The material of the belt for power transmission must be strong in tension, yet flexible and relatively light in weight. The main materials used for this purpose are leather, canvas, rubber, batala, and so on. Modern high performance belts are designed as multiple-ply belts consisting of two or three plies, each serving a special purpose, for example, tension ply, friction ply, and so on.
FIG. 10.2 Major parts of a pulley
The pulley is mounted on a shaft and is rigidly attached to it either by means of a key or set screw (Fig. 10.3). The endless belt passes over the rim of both the pulleys (Fig. 10.1) and power is transmitted from one shaft to another by means of friction between the belt and the rim. The rim of the pulley is not flat. It is slightly convex and is known as the crowning, as can be seen in Fig. 10.3. The curvature tends to keep the belt in the middle of the rim as the moving belt has a tendency to rise to the highest point of the rim. In case of a flat rim, there may be a chance of the belt slipping off along the side of the pulley. The crown height is generally about 1.5 cm per metre of face width.
The width of the pulley will depend on the width of the belt. Pulley widths according to IS 2122 (Pt I)-1973 are given in Table 10.1.
|Belt Width (mm)||Pulley to be Wider Than Belt Width by (mm)|
|Up to and including 125||13|
|Above 125 to 250||25|
|Above 250 to 375||38|
|Above 375 to 500||50|
C.I. BELT PULLEYS
Most pulleys are generally cast. When the diameter of the pulley is less than 20 cm, a solid web is provided to connect the outer rim with the hub. For larger pulleys, arms of circular or elliptical cross-section, tapering outwards are used to hold the rim and the hub at their respective positions. Since the centrifugal stress developed in the arms of the pulley are maximum at their roots, design calculation suggests that the root cross-section should be larger than that of the tip. The proportion of the arms and other parts of a pulley can be obtained from the orthographic projection views shown in Fig. 10.3. For C.I. pulleys, the outside of the hub and the inside of the rim are slightly tapered to facilitate the removal of the pattern from the mould.
FIG. 10.3 Orthographic projection views of a pulley
When the casting of the pulley has cooled down, it contracts, resulting in unequal stresses due to the stress differential, and cracks may develop at or near the junction of the arm and the rim (where there are sharp corners). This is specially true for straight arm pulleys. In case of a curved arm, yielding takes place before breaking and crack formation is thus avoided. Therefore, particularly large pulleys are always made with curved arms.
Fig. 10.4 shows a cast iron belt pulley with curved arms. Apart from the usual pulley, there are some special purpose pulleys available to serve particular requirements. These are discussed below in brief.
FIG. 10.4 Pulley with curved arms
SPEED CONE OR STEPPED PULLEYS
Generally, the speed of the driving shaft remains constant as it is coupled to a motor. Frequently, it may be required to run the driven shaft at a different speed. In such situations, two speed cone pulleys are mounted on two shafts (driver and driven) and through belt, power is transmitted from the driver to the driven shaft. Depending on the position of the belt on different steps of the cone pulleys, the driven shaft speed can be changed although the driver shaft runs at a constant speed. Since the same belt has to be used for all pairs, the diameters of the steps must be such that there is no appreciable change in the belt length.
FIG. 10.5 Stepped pulley developed in solid model
Fig. 10.5 shows a four-speed step pulley made of cast iron. The pulley is hollow inside to reduce its weight. The tap holes, eight in all, indicate that the pulley is mounted and secured on the shaft by a set screw, and placed equally around the circumference. The object is developed in solid model. The dimensions of various parts of the pulley can be obtained from the orthographic views shown in Fig. 10.6.
FIG. 10.6 Half-sectional view of the step pulley
When the size of the pulley becomes too large or it has to be mounted on a long shaft with an inaccessible end, it is convenient to use a split pulley. It is made of two halves that are bolted together at the hub and are secured on the shaft by a key as shown in Fig. 10.7.
The arms are of steel with a circular cross-section. They are shrunk fit into the hub at the time of casting. The other ends pass through the steel rim and are riveted. A coller with a larger diameter is provided at the outer end. It presses the inner side of the rim. Finally, the two halves of the rims are secured together by But Straps, riveted to one half and bolted to the other alternately. The rim is machined after riveting and the pulley is finally balanced. Fig. 10.8 shows an enlarged view of the rivets for better understanding of the joint.
FIG. 10.7 Split pulley
FIG. 10.8 Enlarged view of the joining plate
Even till a few years back, rope pulleys were extensively used in villages to lift buckets full of water from wells. One or more grooves are cut in the rim to accommodate the cotton rope. The diameter of the rope varies between 25 mm to 50 mm. Usually, the diameter of the pulley is kept 30 to 35 times the rope diameter. In some cases, such as in hoisting applications, elevators, and so on, steel ropes are also used. Their length of service depends upon the design of the pulleys around which the rope passes. It is found that a small increase in the pulley diameter has a beneficial effect on the steel rope. Fig. 10.9 shows views of a typical rope pulley carrying two ropes. To highlight the details of the groove along with rope, an enlarged view is also created in paper space environment (Fig. 10.10).
FIG. 10.9 Rope pulley
FIG. 10.10 Enlarged view of the rim
In V-belt pulleys, one or more wedge-shaped grooves (V-grooves) are provided on the rim of the pulley to carry the belt with a V-shaped cross-section. V-belt pulleys are extensively used in industries for their high efficiency in power transmission. Loss due to slippage between the belt and the pulley is considerably less in V-shaped pulleys compared to flat-belt pulleys. The endless belts are specially made of rubber and fibre to withstand high tensile force.
Two views of a V-belt pulley with three V-grooves are presented in Fig. 10.11. Details of the V-grooves along with the belt section is also included in the figure for better understanding of the operation. In general, a belt angle of 36° is selected, since an angle less than 20° would cause self-locking. The latter belt would operate with a lot of jerking and little efficiency. The groove angle must be adjusted in relation to the pulley diameter also.
Fig. 10.11b shows specific application of V-belt pulleys in which motion is being transmitted from the motor shaft (not shown) to the crank that generates the reciprocating motion of the ram of a shaping machine.
FIG. 10.11 V-belt pulley
The cross-sectional dimensions of V-belts are standardised by manufacturers and designated by capital letters such as A, B, C, and so on. Table 10.2 shows the cross-sectional symbols along with dimensions in mm of the cross-section. The pulley may be created in solid model environment. The critical steps are specified so that you can learn how to build engineering components using AutoCAD solid shapes.
|Cross-sectional Symbol||a (mm)||b (mm)|
MAKING A SOLID MODEL OF A V-BELT PULLEY
In the following example, we shall explain step-by-step how a solid model of a V-belt pulley can be generated. The main command used in this exercise is Revolve. (discussed earlier in Chapter IX).
FIG. 10.12 Profile outline of the pulley
FIG. 10.13 The profile of the pulley in SW Isometric view
Step 1 Creating the profile of the pulley in polyline We shall start the drawing in the default XY plane of the computer screen, taking the default limit values. For this drawing, we shall use separate layers for i) polyline outline, ii) solid object, iii) construction geometry, iv) dimension, and so on. For convenience, we shall use the Zoom, Pan, and 3-D Orbit commands as and when required. For using the Revolve command we need an outline profile of the object drawn in polyline and an axis around which the profile will revolve to generate the solid model. Depending upon the degree of rotation covered, we can vary the solid model from a fully revolved solid to a partially revolved solid which will actually give us a sectional view of the solid model. So the first step here is to draw the outline profile of the pulley in polyline. We shall take the measurement from the sectional view of the pulley shown in Fig. 10.11 and draw the outline. The drawing will look as given in Fig. 10.12. We have also drawn the axis line AB (in geometry layer) around which the profile will revolve through 360° to generate the solid model of the pulley.
Step 2 Using the Revolve command First, change the view direction to SW Isometric from the View menu (Fig. 10.13). Now revoke the Revolve command and select the Object option for demarcating the axis of rotation. Then, click line AB as the axis and enter the value of the angle of revolution as 360°. The polyline profile will revolve around the axis AB to generate the solid model of the pulley as shown in Fig. 10.14. We may now adjust the view a little to give us a convenient position to work further.
FIG. 10.14 The solid model of the pulley after the Revolve command and adjustment of view
Step 3 Construction of the through holes and keyways In this step, we shall first construct five holes (four at the web and one at the hub) by means of the Subtract command. We shall also create a keyway by subtracting a rectangular solid element from the central part of the pulley. To do this we have to position the UCS correctly with the right orientation. We shall use the Origin option of the UCS command keeping in mind that there is a difference of level of 20.5 mm (63 – 22/2) between the outer surface of the hub of the pulley and its flat base part through which the four holes of 35 mm diameter will pass. First, we have created the cylinders and the keyway box and then positioned them at their respective locations. We have subtracted them from the solid pulley to obtain the final form in wireframe as shown in Fig. 10.15.
FIG. 10.15 Solid pulley
Step 4 Filleting the sharp edges In this last step we fillet one of the sharp edges of the pulley, that is, the outer edge of the central hub (Fig. 10.16). Readers are advised to try and fillet the other edges to give the pulley its final form.
FIG. 10.16 Solid pulley after filleting
- Draw two views of the pulley shown in Fig. 10.3.
- Develop the solid model of the pulley described in Question 1.
- Why are large pulleys always made of curved arms?
- Develop a solid model of the stepped pulley shown in Fig. 10.5. Take the necessary dimensions from Fig. 10.6. Find out the mass of the pulley.
- Generate the sectional front view and a side view of the stepped pulley from its solid model created in Question 3.
- Prepare a neat and dimensioned sketch of a split pulley having overall diameter of 600 mm and mounted on a shaft of 60 mm diameter. Also show the enlarged view of the joining plate in a separate viewport.
- Develop the solid model of the rope pulley shown in Fig. 10.9.
- Generate the orthographic view from the solid model developed in Question 6. Also show the enlarged view of the rim along with rope.
- Draw a neat and dimensioned views of a rope pulley 2 mm diameter and carrying three 36 mm ropes. Also show an enlarged view of the rim with the rope.
- Draw three views of a 250 mm diameter V-belt pulley carrying four belts and mounted on a shaft having 25 mm diameter.
- Find out the mass of the V-pulley formed in Question 9 considering it to be made of C.I.
- A pair of pulleys is transmitting power from one shaft to another. The diameter of the bigger pulley is 600 mm and that of the smaller pulley is 300 mm. Develop the entire system in solid model.
- Generate the orthographic views from the solid models described in Question 11. One of the views should be in full section.