Design of a drive belt system Essay Example

Design of a drive belt system

Introduction

Belts are commonly used in delivering power in machinery from on point to another most suitably for long distance. Before choosing a belt for a given use, it is important first to analyse its suitability for that purpose. It should be considered that that there are various drive belt types and each can be suitable for a particular application. The critical issue in designing any belt drive is ensuring that it can withstand the applied parameters including torque, tension and the high speeds involved. It is also required that the belt drive meets the highest power transmission efficiency. In this study, I will design a drive system that could be used to transmit power from a motor. The input power will be 10kW and the system should be able to attain a speed of 800rpm. The desired speed reduction ratio is 2.8:1.

Calculations

Since we are considering a normal torque and light shock condition, the Design power will be;

Pd=Pi x Ks= 10kW X 1.1 = 11KW

The available data include;

The length of the belt can be computed as;

= length of the belt in (mm) _bl

= pulley (sheave) diameter(mm) _fd

= pulley (sheave) diameter motor (mm, inches) _md

= minimum distance between the centres C

Lb=4042.04mm This is the same as the pitch length for v-belts. Given by,

Lp=2*1200 +3.142(267mm + 747.6mm)/2 +(747.6mm-267mm)^2/4*1200

Lp4042.04mm

The next step is determining the diameters of the pulleys.

The given variable is the centre distance (550-1200mm) thus the pulley diameters are selected based on the relation d+D=C

1200mm minimum distance between centres is considered. From tables, the standard diameter of the motor drive pulley for the given parameters is 267mm.

Therefore, Diameter for the driven pulley=267x 2.8=747.6mm (CHILDS, 2004).

Belt Velocity

The velocity at which a belt travels may be expressed as

v = π d_m N_m            

v =velocity of belt (m/min)

N_m = rotation per minute of motor (rpm)

v = 3.142 x 0.267m x 800 =671.13m/min   this is approximately 2237ft/min.

The V-shaped belt is, therefore, ideal for such speeds.

Choosing an appropriate V-belt type            

Fig: standard v-belt sections

Fig: table of various v-belt parameters

The design, in this case, will be operating at about 14.8hp. Thus, the appropriate v-belt specifications will be;

a=21/32 in17

b=7/1611

Determining the inside circumference, Li

Li = Lp – q

Where q is a quantity based on the type of v-belt chosen which for this case is 1.8

Therefore, Li = 4042.04-1.3=4040.76mm

Inside circumference, Li

Fig: some inside circumferences for standard v-belts

For this case, we take the values for the already chosen section B i.e., 42in

Determining the correction factor for contact angle, k1

Considering the standard values for values of angle of contact,

For this design, D-d/C = (747.6mm-267mm)/1200 0.4

This translates to a friction factor of 0.8

The belt’s length correction factor, k2

Fig: Belt lengths correction factor

From this table, a length factor for this case of 159.14in is 1.15

Determining the power ratings of standard V-belts, Pr

Fig: Standard ratings for hose powers for v-belts

The sheave pitch diameter considered was 267mmmm=10.5in. The approximate torque will be 6.5hp

Thus, the corrected power Pc can be calculated as; Pc = Pr k1 k^2 =4.85 kW x 0.8 x 1.15=4.46kW.

We can then determine the minimum number of belts N

N = Pd/ Pc=11KW/4.46kW  2.5 say 3.

Discussion

It is clear from the calculation results that the proposed design has dimensions within the allowable limits. The desired speed reduction ratio of 2.8:1 is also achieved. Once the number of belts has been calculated, there was need to know the width of the pulleys. To do this, the width of the belt (of B type v-belts) is multiplied by the number of belts N and then adding the spacing between the belts (PAHL et.al, 2007).

In order for the component to fit in the assigned space, the largest pulley diameter was chosen. Thus the, centre distance could be the most possible. These adjustments will ensure that least number of belts will be needed. The centre distance in this case acts as a guideline and can be governed by these relations; (PAHL et.al, 2007).

D2<C<3 x (D1+ D2)

The centre distance chosen for this design is 1200mm (27.24in) and this value is greater than D2 the value of C is also found to be less than the half the size of upper limit. It can be deduced that, there can be no major harm on the specified job due to implementation of this design.

Material selection

Lighter materials are usually easy to handle and to assemble. Due to its lightness and considerable stiffness aluminium is chosen for the pulleys. In fact, aluminium including some of its alloys has densities up to half that of cast iron. Apart from being a light material, aluminium is rust resistance and has high thermal conductivity.

Design and assembly

The included figures are the major parts of the design. The pulley as can be seen on the fig does no have a completely solid cylindrical part but is spooked in order to reduce the material weight and cost. Such parts are better produced by casting and thus no much complex machining will be done.

To ensure that the shaft and the pulley could rotate together, there needs to be a way to do this. The best way to do this would be making a cut on the pulley to fit with an already machined spline on the shaft.

This seems complex and there could be possible problems if the splines failed. The best alternative is thus using a keyway. A key-way system is easy to manufacture and no much further manufacturing of the parts. Insertion of the the key is done when the key-ways on both shaft and the pulley are in line.

Shoulders are also included on one of the sides where the pulley is resting on the pulley. This would prevent the pulley from sliding along the shaft. The other part of the shaft will be threaded to allow securing of the nut (CHILDS, 2004).

Fig: Key and key way

Fig: top view

Fig: V-belt (B-type)

Conclusion

This study entailed the design of a belt drive system to deliver power from a smaller pulley attached to a motor to another pulley. The design process took into consideration various analysis and calculations. Based on the calculations, a v-belt was selected as the most suitable. Further analysis is done to determine other parameters such as the belt tension. The study objectives were therefore achieved.

References

CHILDS, P. R. N. (2004). Mechanical design. Oxford, Elsevier Butterworth-Heinemann.

PAHL, G., WALLACE, K., & BLESSING, L. (2007). Engineering design a systematic approach. London, Springer. http://site.ebrary.com/id/10230457.