Quality Scan: Clamp Holding Capacity Standards Matter
By Douglas Colby
Director of Engineering
Madison Heights, MI
By Ken Hagan
Madison Heights, MI
Recently, you may have referenced materials for the purchase of clamps, including viewing the product specifications. You found that the data for clamps varied company to company. So, you ask, when is a 1000-lb (454-kg) holding capacity clamp really a 1000-lb holding capacity clamp? And how is holding capacity defined? What clamp gives me the real holding capacity and reliability I require to meet my application needs safely, and achieve my productivity improvement and cost-reduction goals? The answer: It all depends on how one determines and defines holding capacity.
There are numerous variables that can affect the final clamp holding capacities reported by different manufacturers. These include the design of the clamp itself, the thickness of the clamp material, the quality of the material, the relative hardness of the material, the quality of the rivets, the riveting methodology, the quality of the stamping (cleaner holes mean rivets/bushings fit better and provide a smoother action), and the assembled product's structural rigidity. Any of these could affect the actual holding capacity. In turn, the testing procedures used by different companies will yield variances in the definition of holding capacity. There are some considerations you need to keep in mind before you push a clamp to its published upper limit.
One of the first issues to look at is the positioning of the spindle in relation to the pivot point of the clamping bar during holding-capacity tests. The further out the spindle is located on the clamping bar the lower the holding capacity. Depending on the clamp, the position of the spindle can affect the determined holding capacity by as much as 50%.
A second point to consider is that many companies reverse-engineer what others have done. The problem with many reverse-engineered designs is that there is often no testing done to support the product's claimed capacity. Because the product design was copied, so are the data from the original design.
Third, and very important to understand, is the definition of holding capacity. Each company typically (but not always) publishes its defined holding capacity in its product catalog. But you should read the individual definitions carefully. Some clearly indicate that the holding capacity is the clamping load the clamp can withstand before there is no permanent deformation of the clamp components, providing safe and repeatable toggle action. Others indicate that holding capacity is the clamping load at which there is distortion of the clamp that results in failure (unlocking).
Taking holding capacity testing a step further, we include a safety factor in our clamp capacity rating. We define deflection as the amount of distortion the clamp undergoes when subjected to a clamping load. This means that when the clamping load is removed, the clamp will not return to its original shape, at which point the clamp integrity has been compromised. Additionally, permanent deflection is defined as the amount of permanent set the clamp takes when subjected to a clamping load.
To rate the clamp, we take it to the point where the clamp begins to yield. We then use a percentage of that value as our catalog clamp holding capacity.
The lack of communication on standards has brought us to you to ensure that you understand what questions to ask, the individual testing that has occurred, and of course the answer that best meets your goals. In the case where you think you are going to push the upper limits of a clamp's stated capacity, make sure you understand how the capacity was determined. Is the capacity based upon where the clamp begins to yield or where you actually have failure? If it's the latter, pushing a clamp to its limit could have severe consequences to your workforce, to the integrity of the product you manufacture, to your manufacturing output, and to your costs.
This article was first published in the March 2004 edition of Manufacturing Engineering magazine.