We filed for the very first patent on interchangeable heads in 1956. At that time our design was built around a dovetail fixture. That system had a spring loaded pin embedded in the dovetail and that pin kept the interchangeable head in place. The dovetail design helped ensure that torque was delivered or applied at a 90-degree angle.
While we have made some design enhancements over the years our first design still holds because it is still the best way to ensure accurate torque application and eliminate any variance that can be sourced at the interchangeable head.
Changing our heads is fast and simple. All of our heads fit on all of our wrenches. One simply need to insert a CART tool or a small flat head screwdriver into the access hole on the head, depress the pint, and remove the head. To install the new head on the wrench slide the head one to the dovetail, depress the pin and slide the head the rest of the way on the dovetail. You will hear and feel the pin expand into the recess designed into the head.
More than sixty years after the first interchangeable head for torque wrenches went into production one of our customers came to us with an interesting observation and request. Their concern was that in one part of their operation the operators were constantly changing heads and they couldn't meet their production goals because it took too long to change the heads.
As a result, we invented a new system of keeping the heads firmly attached to the dovetail without the pin and spring assembly. Rather than a spring, we used a ball bearing. The need to insert the screwdriver was replaced by simply using your thumb to push the head either on or off the dovetail.
The image on the right shows both systems. While the differences can seem almost minute and difficult to see, the impact is huge. The wrench at the top has the Quick Release ball bearing and the bottom wrench has the dovetail.
As always we are concerned about the structural integrity of any assembly. Based on the force required to withstand torque applications of more than 250 ft-lb we limited the Quick Release Series to 250 ft-lb. If you are using small interchangeable heads (less than 3/4" open end, or square drive, the Quick Release option may be viable. However, if you are using heads larger than 3/4" or 20mm, we strongly recommend the pin and spring dovetail.
Why? Safety. There is always a trade-off between convenience, speed, and safety. The Quick Release dovetail uses a ball bearing to center and hold the head in place. Given the weight of a 20mm head, or a head larger than 3/4", the forces start to approach the amount of force required to remove the head. Violently shaking the wrench could cause the head to disconnect from the wrench. That is a safety concern.
The Quick Release dovetail is perfect for low torque, small head size applications. When ordering the Quick Release dovetail, please be conservative in your assessment.
Combining the required force of application as well as the weight of the head, we prefer to err on the side of safety and structural integrity. That is why we prefer the pin and dovetail head.