In a chain, just because of dragging with a load , the contact surfaces of the joints wear out bringing to a variationdelta p of the theoretic pitch p, wheredelta p is the shifting of the pin axis in comparison to the bush axis.
Therefore, the transmission doesn’t function with an original diameter equal to the theoretic one “d” but with a wider one:
Wear and Tear: Normally, when the selected chain is able to support , without buckling , the draught imposed by the transmitted power , one of the factors that limit the life of the chain is the wear and tear of the working surfaces “pin-bush”.
A chain is considered worn out when the stretching caused by the wear and tear of the contact surfaces “pin – bush” becomes excessive and prevents the proper coupling of the chain with the transmission spur wheel. As a matter of fact, when the stretching exceeds certain values , chain roller, during the coupling with the tooth of the spur wheel guided by the part of the chain not under stress , is inclined to fall on the tip of the tooth instead of in the groove of the next two teeth. Then the chain tends to skip a tooth on the spur wheel. This fact causes high dynamic chain stress and therefore, when the stretching reaches certain values, the chain must be replaced in order to avoid chain breakage. It’s known that most of the transmissions are planned with the intention that the chain life stops when it’s worn out and not because one of its components break.
The above mentioned considerations make us realize that the total draught borne by the chain must be such as to enable the achievement of a certain percentage of stetching caused by wear and tear , in a predetermined lapse of working time.
Normally, the wear and tear of the chain is caused by the rotation of the pins with respect to the bushes , by the rotation of the rollers with respect to the bushes and by the rolling of the rollers along the profile of the spur wheel teeth. There are many factors that determine more or less wear and tear of the chain and among these factors which must be taken into consideration , the main one is surely lubrication.
Special care must be taken during the assembling, lubrication and maintenance of the chain transmission.
Replacement: If a total 3% stretching in the length of the new chain takes place, regular functioning becomes difficult and therefore, it’s convenient to replace the chain because if it stretches too much, it also makes the contact spur wheel teeth, still in good condition, wear out.
On the other hand, a new chain quickly wears out if the gears are already ruined , therefore, the best thing to do is to replace the entire group and more precisely, chain and gears.
The maximum working temperatures normally supported by roller transmission chains are:
- For chains in carbon steel: 180°C, approx.
- For stainless steel chains: 430°C, approx.
The conditions of the environment in which the transmission must work, must be taken into consideration during the planning phase. In fact, the presence in the air of dust, sand, abrasive material or corrosive substances , determine the choice of the appropriate lubricant , as well as the need of an eventual protection of the transmission by means of a carter.
The transmission roller chains must always be properly oiled. This condition is even more important in the event that they are working in a corrosive environment . In fact, in these cases, the lubrication protects the chain against oxidation.
In a highly humid environment, in the presence of acids or alcoholics , stainless steel chains , cadmium plated chains , zinc-chromated and nickel-plated chains can be used.
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