How to size shafting
The most common cause of pulley failure is undersized shafting. An undersized shaft causes excess shaft deflection at the pulley hub. The excess deflection causes the end disk to “flex” back and forth as the pulley rotates. Over time, the flexing will lead to fatigue failure of the end disk; a crack develops in the end disk just outside the weld that joins the end disk to the hub. A larger diameter shaft will lessen the amount of shaft deflection and lengthen pulley life. The Conveyor Equipment Manufacturers Association (CEMA) established CEMA Standard B105.1, “Specifications for Welded Steel Conveyor Pulleys with Compression Type Hubs,” which provides guidance on shaft sizing. They offer an electronic version of this specification on their website.
What is shaft deflection?
Shaft deflection is the bending/flexing of the pulley shaft resulting from the loads placed on the conveyor pulley. These loads come from belt tension on both the tight side and slack side of the belt, loads on the conveyor and the conveyor pulley weight. If the loads on the pulley are known, it is possible to calculate the shaft deflection. CEMA recommends not exceeding 0.13° shaft deflection at the hub. Deflection at the hub can be calculated by approximating the shaft as a simply supported beam with two concentrated loads. This method does not account for any added rigidity from the pulley. Use the following equation:
θ=180Wa(l-2a)/4πEI
Where:
W = total load on pulley in lbs.
a = distance from bearing center to hub center
l = distance between bearing centers
E = modulus of elasticity (for steel this is 29 x 106 psi)
I = moment of inertia. For a shaft this is D4/64 where D is the diameter of the shaft in inches
Θ = angle of deflection at the pulley hub, expressed in degrees.
π = a physical constant approximately equal to 3.14159
If the answer (Θ) is greater than 0.13°, then the shaft may be too small, increase the shaft diameter and recalculate.
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Balancing
Balancing is a very relative concept - all pulleys are balanced to some level, but is that level of balancing adequate for the conveyor application in which the pulley is installed? Only the OEM can determine what would best fit the application.
There are two styles of balancing:
Static balancing – weight is applied to the pulley so that it would stand “still” in any position. The pulley is balanced, but only in a non-moving state.
Dynamic balancing – weight is applied to the ends of a pulley to achieve a specified maximum imbalance at a given rpm. The product is balanced on both ends such that imbalance caused by the rotation of the product is reduced to a specified level.
The International Standards Organization, ISO, published Standard 1940/1 "Balance Quality Requirements of Rigid Rotors," which has been adopted by the American National Standards Institute, ANSI, as S2.19-1975. This standard provides more detail about selecting balancing grades. |
