Tolerances & Other Head Scratchers

The subject of tolerances as they apply is one of the more interesting topics of discussion a Spring Engineer can have with a customer.  One has to remember that customers are end-users and not spring experts.  If the customer is one that sells product on an international level, then there is a chance th.at the customer has Engineers devoted to spring design. Depending on the spring’s importance to the function of the application, the customer’s knowledge of spring science can run the gamut from a thorough understanding, to none whatsoever.  This means the Spring Designer has to be ready to answer questions on virtually any topic related to design, heat treatment, material options, coatings, etc.

Tolerances are a subject that are a careful balance between customer requirement and machine capability.  It’s the job of the Spring Engineer to determine the true needs. The path typically starts with the Engineer telling the customer what the guidelines are based on industry standards such as those of the Spring Manufacturers Institute (SMI).  If the spring function is not critical, then SMI tolerances are usually fine. However, if the application rests highly on the rate or loads of the spring, then special methods and testing may be needed. It’s very important to understand if tolerances shown on the blueprint apply to springs or if they are more appropriate for machined or stamped parts.  It is very common to see tolerances in a tolerance box that apply to stamped and machined parts, but do not apply to springs.  Springs are formed from elastic material that has inherent differences from lot-to-lot. The tolerances need to take these variables into play because the bill of materials for springs is usually just a material, and that’s it. The tolerances can only be determined once the true requirements are established.

For example, a spring that has only dimensional requirements (wire size, diameter, coils and free length) will need to be toleranced on every dimension so the spring maker understands his/her limits off the machine-no parameter is referenced or approximate because it doesn’t apply.  If rates and loads are needed to be held within a degree of accuracy, then some parameters may need to be approximate to fine adjust. If a precise load tolerance is required, then the free length should be negotiated as approximate so quick adjustments in free length (due to wire size variations) will solve the problem and let the spring maker produce consistent product for the critical load. This scenario is an elegant fix for the load variations because the free length is seldom important since most springs are installed to a preloaded height-the free length is essentially moot.

Tolerances need to be appropriate for end-use. Making a customer understand that his allowance of an approximate free length for load adjustment is a win-win scenario.  It is common perception that making something a referenced dimension is not a good practice.  For elastically formed products, it is the lifeblood of efficiency because the spring maker produces conforming quality product more quickly with simple free length adjustments, which reduces set-up time.  That reduced time means less time to get to the customer’s dock, not to mention consistent product from order-to-order.

 

Spring Fundamentals …Use Spring Rate to Determine Valid Load Tolerances

The most misunderstood tolerance percentage I encounter is for loads.  There is this permeation throughout manufacturing in general that a ±10% tolerance on spring load is “standard”.  This is simply not true.

Loads are a direct result of the spring rate and tolerancing must be based on the resultant deflection that is calculated by dividing any given load by that rate.

Load tolerance in deflection (what the spring maker must hold)=load tolerance/rate.

When an order fora spring is sent to the shop floor, it may show a load requirement at a height-for example 125# ±10% at 4.575″ height.  The spring maker does not adjust for load, directly. Corrections to load(s) are done by adjusting the length of the spring, which then equates directly to load. The load tolerance in this case, is ±12.5#.  If the spring rate is 33.3 #/in, the amount of allowable tolerance is ±12.5/33.3 or ±0.375″. But, if the spring rate is 780 #/in, the amount of allowable tolerance is 12.5/780, which is ±0.016″.  This is a huge difference in capability, even though the tolerance percentage was ±10%. If the spring rate is left out of the equation, the 10% rule is nothing more than a shot in the dark.

I have stated in a past article that the rate is the boss. The rate is the determinant of toad potential. The rate is also the determinant of tolerance capability. With this in mind, a ±5% load tolerance may be possible, or it may require a ±25% load tolerance, which is all determined by the capability of the coiling machine to manage its own inherent variability, or to 100% test springs to eliminate the out-of-tolerance parts and the customer pays for that service due to precision tolerances.  Either way, the rate is the key to tolerance capability since it alone defines load variation in any given amount of movement.

 

By: Randy DeFord, Engineering Manager Mid-West Spring & Stamping