Other types of drive

Whilst most fan drives have for many years been of the vee rope type, it should be noted that interest has also recently been shown in other types.

Flat belts

These have improved tremendously and now incorporate syn­thetic tension members having great shock absorbing capacity, strength, suppleness, and dimensional stability.

The high coefficients of friction enable large power to be trans­mitted, but care must be taken in selection to minimise bearing loads. Efficiency can be as high as 98%. With the light weight, centrifugal effects are small and there is not permanent stretch so that tension adjustment is rare.

Toothed belts

These incorporate optimum grades of neoprene with glass fibre tension cords and nylon facings giving considerably improved lives with the new tooth profiles now used. As they do not rely on friction, tensions are lower and therefore bearing loads are lower.

Once installed they do not require re-adjustment, but must be carefully aligned to minimise wear. At start-up under conditions of rapid acceleration, high transient tensions can result due to

Other types of drive

Other types of drive

Figure 11.6 Cross-section of banded belt and pulley rim

When using banded belts it is important that the correct groove profile is selected. The groove spacing i. e. dimension “e” is given in Table 11.5.

Belt section

Groove spacing e (mm)

SPZ

12.0

SPA

15.0

SPB

19.0

SPC

25.5

Table 11.5 Spacing of grooves for different belt sections

Raw-edged vee belts

It was noted in Section 11.4 that both classical and wedge type vee ropes consist of a tension member contained with a rubber base and surrounded by a fabric cover. Of recent years it has come to be recognised that the fabric at the sides of the rope could be deleted without affecting the strength, particularly with the improved wear properties of modern synthetic rubbers. This gives a so-called raw edge and leads to greater flexibility in the belt. Reduced pulley sizes are possible and better wrap is achieved. Greater drive efficiencies are also attained.

This revolution in drives has led to the Standards being out­dated, such that the purchaser is strongly recommended to consult a reputable manufacturer for an up-to-date selection of any drive. As with all such advances, it may take some time for the Standards to “catch up”.

Other types of drive Other types of drive Other types of drive

These are now rarely used for fan drives, due to their limitations in speed and power. There is also a need for lubrication and maintenance, beyond that required for vee ropes.

A chain may be regarded as a belt, built up of rigid links, which are hinged together in order to provide the necessary flexibility for the wrapping action round the driving and driven sprockets. These sprockets have projecting teeth, which fit into suitable re­cesses in the links of the chain and thus enable a positive drive to be obtained. The pitch of the chain is the distance between a hinge centre of one link and the corresponding hinge centre of the adjacent link. The pitch circle diameter of the chain sprocket is the diameter of the circle on which the hinge centres lies, when the chain is wrapped round the sprocket.

Types of chain

There are two types of chain in common use for transmitting power, namely:

• the roller chain

• the inverted tooth or silent chain.

The roller chain. The construction of this type of chain is shown in Figure 11.7. The inner plates A are held together by steel bushes B, through which pass the pins C riveted to the outer links D. A roller R surrounds each bush B and the teeth of the sprockets bear on the roller. The rollers turn freely on the bushes and the bushes turn freely on the pins. All the contact surfaces are hardened so as to resist wear and are lubricated so as to reduce friction.

Figure 11.8 (a) shows a simple roller chain, consisting of one strand only, but duplex and triplex chains, consisting of two or three strands, may be built up as shown in Figure 11.7 (b), each pin passing right through the bushes in the two or three strands.

The inverted tooth or silent chain. The construction of this type of chain is shown in Figure 11.8 (a). It is built up from a se­ries of flat plates, each of which has two projections or teeth. The outer faces of the teeth are ground to give an included an­gle of 60° or, in some cases, 75°, and they bear against the working faces of the sprocket teeth. The inner faces of the link teeth take no part in the drive and are so shaped as to clear the sprocket teeth. The required width of chain is built up from a number of these plates arranged alternately and connected to­gether by hardened steel pins which pass through hardened steel bushes inserted in the ends of the links.

The pins are riveted over the outside plates. The chain may be prevented from sliding axially across the face of the sprocket teeth by outside guide plates without teeth, or by a centre guide plate without teeth which fits into a recess turned in the sprocket.

Other types of drive

— b

( • )

Other types of drive

( a )

Figure 11.8 Details of inverted tooth chain

Figure 11.8(b) shows the type of hinge used in the Morse silent chain. This reduces friction by substituting a hardened steel rocker on a hardened steel flat pivot for the pin and bush.

When the chain is new, the position which it takes up on the sprocket is shown in the upper part of Figure 11.9. Each link, as it enters the sprocket, pivots about the pin on the adjacent link which is in contact with the sprocket. The working faces of the link are thus brought gradually into contact with the correspond­ing faces of the sprocket teeth. A similar action takes place as each link leaves the sprocket. Hence there is no relative sliding between the faces of the links and the faces of the sprocket teeth.

Other types of drive

Figure 11.9 Sprocket and silent chain

As wear takes place on the pins and bushes, the smooth action of the chain is not impaired, but the chain rides higher up the sprocket teeth and the effective pitch circle diameter of the sprocket is increased, as shown in the lower part of Figure 11.9

Standards for chain drives

The Standards for chain drives are not nearly so comprehen­sive as those for vee belts. However, the ISO standards given in Section 11.7 Bibliography, are relevant:

Efficiency

Many of these alternative drives have been designed to over­come some of the shortcomings of the standard vee rope drive.

Normal belts suffer from tension decay, resulting in slip and loss of efficiency. They require frequent adjustment to maintain per­formance. Being a single member, these alternatives do not suffer from matching problems. In a multi-belt drive, where there is a variation in length, however small, the shorter belts will be under tension and transmitting the power whilst the lon­ger belts are running slack and contributing little. Effectively the drive is under-designed and will have a short life.

Other types of drive

0 50 100 150

Power % of rating Figure 11.10 Efficiency of toothed and vee belt drives

I…. I I I I I I I

Higher fan speeds tend to have higher losses

Than lower fan speeds at the same power

S

N

S

S

X

X

..

V

100 80 ; 60

! 40

! 30

2

1.5

1

IA <D IO O ^

Ј <NJ O

Motor power output (kW)

Shafts are parallel and aligned but p Jleys are not aligned

подпись: shafts are parallel and aligned but p jleys are not aligned

Shafts are not parallel to each other

подпись: shafts are not parallel to each other
 
Figure 11.11 Estimated vee belt drive losses

Drive efficiencies can be maintained over a wider range of pow­ers and can in any case exceed the 97% possible with vee ropes. It should here be noted that if a vee drive is either under — or over-engineered, efficiency will suffer as shown in Figure

11.10.

With very small drives, the difference in power transmitted be­tween, say, one and two belts or between two and three belts, is obviously substantial. The chart in Figure 11.11 has been based on AMCA International data and may be used to esti­mate the losses in a standard vee belt drive. Such losses will need to be added to the fan power to determine the power re­quired from the motor.

Example 1 Motor power output Pm is determined to be 9.9kW. From curve drive loss = 5.8%. Drive loss P-i = 0.58 X 9.9 =

0. 6kW. Fan power input Pf = 9.9-0.6 = 9.3kW.

Example 2 Fan power input Pf = 0.75 kW. In this case it is nec­essary to estimate motor power input. Motor power output =

0. 88 kW. From curve drive loss = 15%. Drive loss Pi = 0.15 X 0/88 = 0.13. Fan power input = 0.75 + 0.13 = 0.88 kW which is correct.

Posted in Fans Ventilation A Practical Guide


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