2.2.2 Repeated Load Tension, Fatigue Failure
In Basics Section 2.2.1, we looked at the case of rotating chains without load. In this section, we'll examine rotating chains with load, a typical use of chains.
In Figure 2.15, the left sprocket is the driving side (power input) and the right sprocket is the driven side (power output). If we apply counterclockwise rotation power to the driving sprocket while adding resistance to the driven sprocket, then the chain is loaded in tension mainly at the D~A span, and tension is smaller in the other parts. Figure 2.16 shows this relation.
Figure 2.15 A Typical Chain Drive with the Driving Side on the Left
Figure 2.16 Chain Load with the Addition of Resistance
Chains in most applications are typically loaded by cyclical tension. Chain fatigue is tested under pulsating tension via a fixture. The fatigue limit will occur between 106 to 107 times. Figure 2.17 shows the concept of repeated load tension, where Pa represents the amplitude.
NOTE: If the minimum force is zero, the chain is free to move during testing. Therefore, JIS provides Pmin = Pmax × 1/11, as in Figure 2.17.
When a chain that is more than five links and of linear configuration receives repeated load, it can be shown as a solid line (as in
JIS B 1801-1990 defines the breakage load in 5 x 106 times:
Pmax = Pm + Pa = 2.2 Pa
Figure 2.17 Repeated Load Tension
as the maximum allowable load. Figure 2.18 shows one result of fatigue examination in this way. In the figure, the vertical axis is Pmax and the horizontal axis is the number of repetitions. When the repetitions are less than 104 times, the test results fluctuate greatly. Therefore, these figures are practically useless, and are not shown here.
In the previous paragraph, we need to be alert to what the JIS regulation is really saying: "JIS B 1801-1990 defines...Pmax = 2.2 Pa as the maximum allowable load." This is set up with wrapping transmission as a model (as shown in Figure 2.15), and with the supposition that the smaller load side tension is 10 percent of the larger load side tension.
In actual practice, even if we use wrapping transmission, the smaller load side tension may be almost zero; and in the case of hanging or lifting, the chain's slack side also doesn't receive any load. In these cases, the conditions can be shown as a dotted line (Figure 2.17); chain load = 2 Pa′ and Pmin = 0; therefore 2Pa′ < Pmax .
Figure 2.18 Fatigue Strength
If you follow the JIS definition of Pmax as maximum allowable load and you choose a chain on the higher limits of the scale, the chain might not stand up to those strength requirements. In some situations a fatigue failure might occur even though it met the JIS requirement for maximum allowable load.
This is the reason that some manufacturers, such as Tsubaki, use 2Pa as the maximum allowable load; or some manufacturers calculate 2Pa under the situation of Pmin = 0 and show this in their catalog. In the latter method, the 2Pa′ value is larger than the value of the former method. The maximum allowable load value of the JIS method is 10 percent greater than the former method of 2Pa .
In addition, some manufacturers, including Tsubaki, establish a fatigue limit for strength at 107 cycles. JIS sets a fatigue strength at 5 x 106 cycles.
Including the JIS scale, there are more than three ways of expressing the same information in manufacturers' catalogs. Therefore, you should not make a final determination about a chain's functions simply by depending on information found in different catalogs. Consider a manufacturer's reliability by checking whether they have their own fatigue-testing equipment. Ask if they show fatigue limit data in their catalogs. The quality guarantee system of ISO 9000 series is checked by third parties (instead of users) to gauge whether or not their system of quality guarantee is adequate. It would be safe to choose manufacturers who are ISO-9000-series certified.