The simplicity of positive displacement blower drives

The simplicity of positive displacement blower drives

Blowers are dumb beasts.

They do not know nor care what is driving them.  They just rotate and move air.

There are a number of methods of driving a rotary positive displacement blower.

The simplest and preferred method is a simple flexible coupling, driving the blower at the same speed as the motor.  Woods Dura-Flex and Sure-Flex couplings are examples.  They, of course, must be properly sized and rated for the motor horsepower, the load, and the speed.  Certainly the use of a flexible coupling is limited to matching the motor speed.  Since a blower operates on a curve, the optimal blower selection will most likely require an input speed that does not match the motor speed.

When the motor speed does not match the required speed of the blower, a v-belt drive is needed.  A number of factors are used to properly size a v-belt drive, including the motor horsepower and speed, service factor (1.4 for rotary positive displacement blowers), blower required speed and horsepower, motor and blower shaft diameters, and maximum allowable “overhung load” on the blower shaft.  Of equal importance is the location of the blower sheave on the shaft.  In general, the sheave must be placed as close as possible to the fact of the blower, in some cases no further away than 3/8 inch.

The most frequent failure we see in rotary positive displacement blowers is the drive shaft bearing.  This is the bearing on the input shaft on the main rotor.  With a v-belt drive, this bearing is susceptible to the greatest stress of any of the bearings.  In addition to carrying the weight of the rotor and the rotational forces imparted to it, it also has the side load created by the v-belts.  If the belt drive is not properly sized and located, it can place unacceptable overhung load on this shaft and its bearing.

All Gardner Denver blower manuals (Sutorbilt®, DuroFlow®, HeliFlow®, & CycloBlower®), and most, if not all, blower manufacturers includes a section on the drive sizing, with charts and formula to calculate the overhung load and insure it is within allowable limits.  These equations are pretty simple math, but the load data from the v-belt drive must be provided in order to calculate these factors.

V-belt drives for rotary positive displacement blowers should be sized based on a 1.4 service factor.  This ensures the integrity of the drive itself, and makes sure the drive is adequate.  A lower service factor can result in the belts “rolling” and slinging off the sheaves.  A service factor too high can increase the overhung load on the blower shaft and bearing.

A third drive method utilizes both a flexible coupling and v-belt drive.  This is called a jack-shaft drive, with the motor driving through a v-belt drive to a jack shaft (properly sized shaft and pillow-block bearings).  This shaft then drives the blower via the flexible coupling.  In this method, the excessive overhung load on the blower shaft is avoided by direct driving the blower, but flexibility in speed is maintained by use of the appropriately sized v-belt drive.

Pye-Barker Supply Co. has the expertise and resources necessary to properly size the various drives and calculate the resultant overhung load on the blower shaft.  We also have contacts in the industry to assist in designing the proper jack-shaft drive as needed.

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