IDLERS / TENSION PULLEYS
Idlers are basically no loaded wheels that are used in a drive system under the following conditions.
1) Fixed centre distance, so as to provide the required installation & take-up allowances.
2) On long & unsupported spans, as dampers & guides where the chances of vibrations are more.
3) As outside idlers when the arc of contact on one of the drive pulleys is too low. Also helps in reducing the slippage & the need to increase the number of required belts.
4) As guide idlers where the drive system pulleys are not in the same plane.
5) To guide the belts past obstructions.
6) As pneumatically, or spring loaded idlers to maintain the constant tension in the drive.
7) As clutches where the driven pulley can be engaged or disengaged.
The usage of idlers should be as far as possible avoided as they generate additional bending stresses in the belt, leading to drastic reduction in the belt life. However, under the conditions listed above where it may be absolutely essential to use the idlers the following criteria must be observed when designing drive.
1. Idler configuration
2. Position of idler in the belt span
3. Shape of idler
4. Allowance for idler travel
5. Correction of power rating
Principally idlers can be used internally or externally depending on the drive conditions. However, unless the drive requirement calls for outside idler, possibly inside idler should be used. The inside idler can be either flat or grooved pulley depending on the type of belt used in the drive system It is suggested that the flat inside idler be used only when classical section is used while in all other cases a grooved pulley be used. However the usage of inside idler reduces the arc of contact on the loaded wheels and with it consequently The arc of contact correction factor. Hence when desi3ning the drive with inside idler the arc of contact correction factor should be selected for the position of the idler at the point of maximum belt stretch. Refer to table Arc of contact correction factor c2 .
Outside idlers should always be flat one because they run on the back of the belt Outside idlers invariably increase the arc of contact. Care should however be taken to see that the maximum belt stretch is achieved and the contact with the opposite side of the span is avoided. The reverse bending caused because of the usage of the Outside idler reduces the life of the belt.
We suggest the usage of special construction Darwin Plus belts on these drives. Please contact our Technical Department.
Practice has shown that the placement of idler, whether it is an inside idler or an outside idler, should be on the slack side of the drive. This helps in significant reduction in the tension idler force.
In the case of inside idlers, grooved pulley can be placed anywhere in the entire span length on the slack side of the drive. However to obtain the best from your drive it is suggested that wherever possible the arc of contact on both the drive pulleys should be brought as close as possible to each other when the idler reaches its limit position.
Flat pulleys, used as inside or outside idlers are to be placed as far as possible away from the grooved pulley on which the belt runs next. This will avoid any alignment errors between the idler & the pulley and the resultant sideways movement of the belts on the pulley. Refer to the diagrams below.
On drives where inside idler are used to break long spans, it is suggested that a grooved pulley be used because the usage of flat idler can result in transverse vibrations leading to the belt turnover.
Minimum diameter recommended for idlers
Inside Idler > Smallest loaded pulley in the drive system or smallest permissible pulley diameter for section used.
Outside Idler > 1.5 times the smallest pulley in the drive system.