In order to properly size both the outer diameter of a conveyor pulley and select an appropriate shaft diameter, it is important to first understand the pivotal role that selection plays in avoiding the most common cause of premature failure, shaft deflection.
The single largest contributor to premature failure of conveyor pulleys is end disk fatigue caused by excessive shaft deflection. Shaft deflection is the bending or flexing of a shaft caused by the sum of the loads on the pulley. The sources of these loads include belt tension, product load and the weight of the pulley itself. Excessive shaft deflection occurs as a result of an undersized shaft.
Excessive shaft deflection occurs when shaft diameter is improperly sized for the demands of an application.
Although it may appear as a potential solution, selecting a shaft material with greater strength characteristics will have virtually no effect on its stiffness as it pertains to shaft deflection. The Modulus of Elasticity, which is a physical property of a substance which describes its tendency to deform elastically when a force is applied to it, remains virtually the same across all grades of steel, and because of this, the only proper way to increase the stiffness of a steel conveyor pulley shaft is to increase its diameter.
Premature failure of a conveyor pulley is not likely to occur from an oversized shaft, but an undersized shaft can produce harmful and destructive results. The Conveyor Equipment Manufacturers Association (CEMA) recommends that shafts be designed with a maximum bending stress of 8000 psi or a maximum free shaft deflection slope at the hub of 0.0023 inches per inch.
Selection of an outer diameter requires comprehension and consideration of several variables found within the given conveyor system. Pulley diameters and shaft diameters should be selected using tools such as ANSI/CEMA B105.1-2003 (SEE APPENDIX A). The following application variables need to be considered in the selection of both the pulley outer diameter and the shaft diameter:
Most conveyor belt manufacturers recommended the minimum pulley diameter specification for conveyor belting based on the individual belt characteristics such as the belt material, construction and profiles.
The amount of traction between a drive pulley and a belt can be increased by increasing the arc of contact between the two surfaces. The arc of contact, or belt wrap is the angular distance a pulley travels while in contact with the belt, and is measured in degrees. Increasing the area of contact between two surfaces does not increase the coefficient of friction between the two surfaces. As explained by the Euler-Eytelwein Formula, increasing the arc of contact will increase the amount of frictional force between a belt or rope and a round object such as a pulley.
The purpose and position of a pulley in the conveyor (i.e. Drive, tail, bend, or take-up) impacts how much load the individual pulley will experience while in operation. In general, pulleys in the position of driving the conveyor belt will experience greater loads than pulleys in other positions. This is largely due to the increased level of work and tension required of the drive pulley as well as the potential for additional loads produced from the setup of the drive device.
Selection of a proper shaft diameter will take into account the expected service life of the pulley at the anticipated speeds and capacities. In general, if a longer duty cycle is preferred, shaft diameter should be purposefully oversized.
Any slot or groove machined into the outer diameter of the shaft can create stress concentration
points on the shaft. These stress concentrations require consideration of selecting a shaft of larger diameter.
The total weight of the pulley assembly to be supported by the shaft will impact shaft sizing. Selection of a pulley with robust construction and heavier weight should be accounted for when selecting shaft diameter.
The distance between the center of each bearing support and the center of each hub connection will impact the degree to which the shaft deflects and should be accounted for when selecting a shaft diameter. Having a greater distance between the hub centers and the bearing centers will require a larger diameter shaft to accommodate the same load.
When the physical constraints of a conveyor system will not allow you to properly size your shaft diameter, shaft turndowns may be utilized to increase the load capacity of a pulley. A turndown is where a larger shaft is turned down to a smaller diameter at the ends, while retaining the larger diameter through the pulley.
The sudden change in geometry between a shaft major diameter and a turndown is an area of stress concentration. A radius should be incorporated to reduce the stress concentration at this point.
In addition to providing some degree of belt tension to the conveyor system, the load of the conveyed product can also contribute to the load being directly applied to the conveyor pulley. This becomes a more significant factor when the product is being loaded on the conveyor in an area near the pulleys. The greater the amount of load applied to the pulley, the greater the shaft diameter required to properly support the load.