Warm pre-stress operation
There is a general phenomenon for all compression springs: "relaxation". The spring loses a certain amount of its free length and the exerted load, during its work, particularly, when it works quite close to its elasticity limit, and/or at severe conditions, regarding high temperature and intensive operation.
The relaxation, especially when it is relatively significant, might cause malfunctioning of the spring, and sometimes it is critical for its job.
The relaxation extent is directly influenced by the raw material type and spring structure, especially by D/d ratio.
2. Method for prevention relaxation.
In most cases the regular/cold pre-stress operation diminishes the potential relaxation to an acceptable extent. The Goodman Diagram might reflect the area where a spring works at a safety region, and the pre-stress operation clearly shows how much the same spring has better quality after it is pre-stressed.
When the spring is supposed to work at extreme condition, either concerning high temperature or at dynamic surroundings, it is recommended to carry out warm pre-stress operation, in order to prevent load loss, due to the relaxation phenomenon.
The ordinary systems to carry out the above operation are similar and based on the same principles, even though, they are not equal. Nordia springs treats the springs according to the following process:
The operation is carried out as the final one before packaging, when marking for identification and inspection are not considered.
Usually it is carried out after shotpeening operation, or after grinding.
The springs are entered into a conveyor furnace, when they are installed on pins. The velocity of the belt and the temp inside the furnace are determined according to the requirements from the spring, and from its structure.
Immediately after the spring finishes the passing inside the furnace it is circulated into a piston which presses it to its solid height, when the spring is still hot. Immediately after the pressing operation, a liquid cools the spring, and the springs are liberated from the piston.
At Nordia springs in-house process, the treated springs are automatically inspected for the relevant length/load; the springs are sorted automatically for O.K./NO.K springs.
There are several tests to examine the efficiency of the pre-stress operation. Most of the tests are carried out like that: samples from the batch are entered into a furnace, in their deflected state, not far from the solid length. After 10-30 hours the springs are taken out of the furnace and the exerted loads are examined before and after this test. The difference between the loads before and after test should not be bigger than 2-3%.
The warm pre-stress operation diminishes significantly the relaxation phenomenon, and it is recommended to carry out this operation on every spring which its structure or the extreme conditions, which it works in, justifies the cost of this operation. Theoretically this operation is recommended to any compression spring, in order to promise smooth and stable operation during the life of the spring.