OTHER SPROCKET TECHNICAL CONSIDERATIONS
OTHER SPROCKET TECHNICAL CONSIDERATIONS
Other Considerations When determining whether you need replacement sprockets, consider the following important points: Chain Interaction The chain-sprocket interaction is the criterion upon which most users make their judgments about replacing sprockets. If the chain engages and disengages the sprocket smoothly without hanging up or snapping into place, most people will not replace it. If a chain does start to hang up on the sprocket, damaging chain overload conditions can develop rapidly. We suggest replacing sprockets before hang-up develops. Reversible Almost all sprockets are reversible. The key to being able to reverse sprockets is symmetry. If the sprockets are symmetrical from side to side, then they can be reversed. Reversing is not suggested in most circumstances, especially with those applications that wear the sprocket bottom diameter. New Chain We suggest you order new sprockets when chain is replaced. New sprockets ensure proper chain interaction and also provide maximum wear performance. Attachment Clearance Any time an attachment is in the area between, above, or below the sidebars, make sure the attachment does not interfere with sprocket action. Relief Pocket (Mud Relief) In applications where material buildup may be a problem, the bottom of the tooth pocket can be beveled on the side to allow the material to “squeeze” out. This reduction of contact area is not critical because the pressure on the bottom of the pocket is very light in horizontal conveyors. Other relief styles may be necessary for vertical conveyors. Advantages of Using Larger Sprockets Choose the largest sprocket that will fit your application. Small sprockets cause greater shock and consequently more wear on both chain and sprockets. There are several reasons for this. 1. To engage small sprockets, chain must flex more which causes increased rotation of the pin in the bushing. Since this is one of the major causes of chain wear, this flexing action should be minimized. 2. Small sprockets with fewer teeth wear out much faster than sprockets with more teeth. More teeth provide an opportunity to distribute the wearing action. 3. Larger sprockets cause smoother operation because the greater number of teeth will pick up the load more frequently.
Chordal Action Chordal action is a very important concept in sprocket function. A sprocket is a collection of chords, or straight segments, that approximate a circle. The more teeth a sprocket has, the closer the chords are to a circle. The problem with a chordal form is that the lineal output is not consistent. Since the sprocket is not a perfect circle, the distance from the shaft center to the chain center line varies. As this distance varies, so does the lineal output (assuming a constant shaft rational speed). A hexagon inscribed by a circle represents the six-tooth sprocket shown below. You can see that the distance from the center to the corner is different than from the center to the middle of the side. The corner would be the equivalent to the chain joint center, and the side is equivalent to the chain center line at mid-pitch.
Sprockets Keyed-In-Line When two strands of chain run parallel with each other and they are to be timed identically, it is important to have the keyway position to be in the same relationship on both sprockets. This is referred to, and should be ordered as “keyed-in-line” or “matched in pairs” to obtain this feature (Figure 1) . Sprocket Size Use the largest diameter conveyor sprocket that space and economics permit. This minimizes chain speed variations and pulsations and reduces wear to the chain and sprocket. Sprocket Life When sprockets are worn, the chain tends to cling to the sprockets or vibrate. The amount of allowable wear depends on the application type and chain size. Wear to a depth of 0.12 " (3 mm) to 0.24 " (6 mm) is usually a sign that the existing sprocket should be replaced, illustrated in Figure 2 . If the sprocket teeth are worn, the alignment may be incorrect. Proper axial alignment of the sprockets will help reduce or even eliminate wear of sprocket teeth, illustrated in Figure 3 .
Minimum Chain Speed
Figure 2 - Sprocket Wear
Figure 1
V 1 =.2618 (PD) COS (180/T) N
Maximum Chain Speed
Figure 3 - Sprocket Tooth Wear
Amount of Wear
V 2 =.2618 (PD) N Where V1 and V2 in fpm N in r.p.m. PD in pitch diameter, in inches
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