RELIABLE CHAIN | MODEL CONSTRUCTION

Amateur designers of vehicles, and motor agricultural machinery often have to work with what is at hand. Typically, this is the engine from a moped or motorcycle, the wheels of a Bicycle, scooter or sidecar, some factory default or self-made components and parts. When the idea to connect them all in varying designs, the inevitable question what to take counter sprocket (driving is already on the engine), how to calculate?

The simplest and most affordable is the calculation of the proportion. For example, you have the engine from a moped, the diameter of the rear wheel which D, is driven sprockets with number of teeth z And you want to use it on my car complete with wheel diameter d. Therefore, you need to find or fabricate the sprocket with number of teeth Z₂ — it is easily calculated by the following formula:

Z₂ = d*Z₁/D.

The result may be not integer and fractional. It is clear that stars with a half or quarter of a tooth does not happen. Therefore, the value should be rounded to the nearest whole number. But remember that rounding off in a big way, you can win in power (the throttle response is better, the engine will handle the increased load), but lose top speed. With a smaller integer number of teeth will succeed Vice versa.

And again. Don’t forget when calculating to compare between a number of teeth Z₁ leading (on the engine) and Z₂ slave (count) of the sprockets. They don’t have to be a multiple of, that is:

Z₁≠ Z₂*P;

Z₂≠ Z₁*P,

where N is any integer. Or perhaps a periodic summation of the load chain due to the intenseness of the veins, making uneven transfer code, increases wear, increases noise.

In addition to the transfer has not worn out, do not forget this rule: any number of teeth of the sprocket should not be a multiple of the number of chain links, that is:

N≠ Z₁*N ≠ Z₂*P,

where N is the number of links.

Why? Let’s consider an example. For example, the asterisk in the calculations turned 25 teeth and the chain has 100 links. If the combination of any one tooth with a certain link is the largest tightness of the chain, it is the combination of our calculated data will be repeated for every four (100:25=4) of the turnover sprockets. Add to that the natural oscillations of the circuit, appearing at great speed. Thus, rapid wear of the transmission is inevitable.

By the way, to reduce the effect of resonance in the design of the rear wheels injected rubber vibration absorbers, the serviceability of which is to follow.

Let’s continue our calculation. Selecting a different sprocket with 24 teeth, after simple arithmetic can see that unwanted combination will be repeated only once during the 24 full rotations of the chain. It is quite possible to reconcile.

There are other options. For example, it may happen that replacing a star is impossible, but it is possible or even desirable to shift forward or backward the wheel. Then recalculate the length of the chain to reflect the new number of its components and specify the distance between axes of sprockets. This is done using the following formula:

A = t/4 [N – (Z₂+Z₁)/2 + sqrt((N – (Z₂+Z₁)/2)²– 8((Z₂-Z₁)/2*3,14)²)],

where A is the distance between axes, mm;

t — step of the selected chain, mm;

N is the number of chain links.

If the distance obtained is not satisfied, you can recalculate it by taking the other value of N.

But suppose you want to determine chain length in structurally predetermined center-to-center distance. Find it by the formula:

N = 2A_t + (Z₂+Z₁)/2 + ((Z₂-Z₁)/2*3,14)²* 1/A_t,

where A_t — tentative centre distance, taken steps in the chain, that is:

A_t= A/t.

The resulting value rounded to the nearest whole number (taking into account all the above relation) and the formula specify the true distance between axes.

Don’t forget in your design to provide adjustment of chain tension. This can be either a device with a roller or a groove that allows you to shift the wheel axle or guide rail for changing the position of the engine, or something else, ensure the tightness. In any case it is impossible to shorten the chain, wanting to eliminate the sagging when pulling. In the operation of this circuit will be extremely unreliable. Now back down why: violated all mathematical relationships.

A little bit about the operation. As already mentioned, of great importance for reducing the wear of the chain is its tightness. Depending on the suspension design of your car it is necessary to clearly determine in what position the plug chain is most taut, and only then conduct the adjustment. In some cases, the maximum interference occurs when freed from the load of the suspension, in other — when loaded (Fig. 1).

Fig. 1. The maximum chain tension occurs at the lowest (a) or at maximum (b) load on the fork depending on, above or below the wheel axis of the motor shaft and the axis of the pendulous fork:

1 — star wheel 2 — circuit, 3 — way plug, 4 — axis of the pendulous fork, 5 — sprocket.

By installing the hanger to the desired position and slowly rotating the wheel, you can ensure that the chain is slack then stronger, then weaker. This is due both to the uneven wear of the links and possible runout of the sprocket. To adjust the slack it is only necessary when the chain is most taut. If you for some reason do not do this check and tighten the chain as necessary, so at some point she is going to experience congestion, it to wear out quickly herself, and wear and tear on the bearings of the motor shaft and the wheel hub.

It happens that to achieve the optimum tension when the adjustment fails: the chain is too taut, then slack so that jumps through the teeth of the sprockets. In this case it should be replaced, perhaps with an asterisk. Otherwise the new transmission will wear out very quickly, because the teeth of this form (Fig. 2) strive to push the links up and pull the chain.

Fig. 2. The nature of the wear of the sprocket teeth, unsuitable for further use.

Speaking of pulling. Many designers understand that in the truest sense of the word. Say, the circuit load becomes longer. This is incorrect. The chain is not served would go an hour and broke under pressure. Elongation, i.e. the elongation of the chain occurs not due to residual strain, and by wear and tear in the joints of the links, something like the one shown in figure 3.

Fig. 3. Elongation of the chain due to wear of the plates and rollers.

More about the causes of rapid wear of the chain transmission. It would be ideal if you succeed in its design to close the transfer of the protective casing. Statistics show that a well-oiled closed circuit is several times longer open. And it is clear why: dust, dirt and sand, getting on the friction surface as sandpaper skin, “eat” connection.

Described in many of the classic ways to increase the life of the chain method “.” its in a graphite lubricant, greatly reduce wear, but is generally little used in practice because of the complexity and difficulty. Therefore, the majority of Bicycle motor – fans, under various pretexts avoid “.” and lubricate the chain outside. Thus only contribute to the subsidence, mud build-up, and hence intensive abrasive wear of the chain. Consider how experienced the operator, if the chain does not “boil”, it is better to leave it perfectly dry, not lubricating anything.

I think, these simple tips will be useful to anyone who just draws your future car who is not the first year operates a makeshift structure with a chain drive.

V. SILVANOVICH, design engineer, Leningrad

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