One of the most common questions about Constant Force Springs concerns the mounting of the spring on a spool. For starters, the spring doesn’t have to be mounted on a spool; it can be placed in a cavity or on an undersized bushing or pin. There are differences in the performance of each design so let’s take a look at the advantages and disadvantages.
The main advantage of spool mounting is smooth, repeatable operation due to the reduction of friction. Material selection of the spool/shaft interface should be based on an effort to create a bearing surface that will minimize friction throughout the life of the spring.
When a spring is mounted in a cavity the designer will have to manage friction, and the resulting reduction in usable spring force. The advantage of a cavity mount is the ease of assembly. Likewise, when mounted on an undersized pin there will be an audible “click” and drop of load as the inner end of the spring passes over the pin. The thicker the steel the more pronounced this will be. Again, the ease of assembly is the advantage here.
Since assembly is more difficult when placed on a spool we need to discuss this operation next. The diameter of the spool should be 15% – 20% greater than the nominal inside diameter of the spring. This is due to the tolerance of 10% for the inner diameter of a spring. The spring should fit tightly on the spool so we need to account for this tolerance.
One misconception of mounting the spring on a spool is that the spring needs to be attached or fastened to the spool. This is not the case. The spring needs to be long enough to have a minimum of 1.5 wraps remaining on the spool when fully extended. This will allow the spring to stay tight on the spool and retract when released. Therefore, the spring cannot be used as a stop.
The spring inner diameter can be opened up by securing the inner end and pulling back on the outer end. The spool can then be slid into place. For longer and heavier springs it is sometimes best to reverse the spring, making the outer end the inner end. Once the inner loop is exposed it can be placed over the spool and then re-wound onto the spool in the correct orientation. When performing this operation it is necessary that the spring is not back bent (forcing the spring against its natural curvature). In some cases the spring can be produced inside-out so the initial reversing is not necessary. The inner end of the spring as received will be the outer end when wound onto the spool.
Here at Vulcan Spring we are pleased to assemble springs onto our customer’s custom made spools. There is no need for the customer to perform this assembly step. The customer can receive the spring mounted onto their spool and then simply slide the assembly onto a shaft of their design. This will save considerable time and money.
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