As you might expect, we discuss torque with our customers every day. People certainly have a lot of questions about how to measure torque. They want to make sure that the proper torque is applied to the cap. So naturally, many people who are new to caps want to find a way to directly measure the torque output of their capping machine. The ideas they usually try are logical and well thought out, but unfortunately, they can lead to inaccurate results. Our purpose here is to explain why.
Many people understandably assume that if they put a bottle into a cap torque tester and then tighten the cap with a capper, the tester will tell them what torque the capper just applied to the cap. Unfortunately, this is not the case. If the cap is screwed onto the container very slowly, the cap torque tester will only measure the torque applied to the cap. A capping machine can tighten caps very quickly. The sampling rate (how often the cap torque tester reads the torque output) of the cap torque tester is too low to accurately measure the cap torque applied by the capping machine.
Another approach that people sometimes follow is that they try to measure the torque output of the capping motor itself. They conclude that if the torque output on the motor shaft is a certain amount, then they must apply the same amount of torque to the cap. This method also produces inaccurate results. Below we will explore this method in more detail and explain why.
Explain why the torque output of a capping machine cannot be measured directly
Along with the explanation of why, we will also provide you with a practical example. For this particular application, our capping machine provides us with an application torque specification of 27 to 33 inch pounds. So if we apply 27 to 33 inch pounds of torque, the cap will seal correctly on the bottle.
But how do cap manufacturers meet application torque specifications? The process they perform is not widely known and therefore is the source of much of the confusion on the subject.
When manufacturers of caps and bottles perform tests to determine application torque specifications for specific caps and bottles, they follow the Plastics Industry Association Plastic Bottle Institute (Technical Bulletin PBI 7). The key part to understand is how the test is performed.
The test is performed by clamping the bottle in a cap torque tester and then gradually and very slowly applying the cap to the selected torque. They then check the seal on the bottle. By repeating this test several times at different torque amounts, a range of torque values was found in which the threads on the cap and on the neck of the bottle escaped in a way that did not allow any contents to join the bottle, nor did it allow any external material to enter the bottle. This range of torque values becomes the specified application torque range.
The key part to understand is that the application torque specification is achieved by slowly and gradually tightening the cap. Conversely, capping machines are used to increase the productivity of the capping process. As a result, capping machines tighten caps very quickly. The difference in speed between the two methods is the reason for the difference in the final torque applied by each method.
When capping a bottle with a capping machine, two forces are at play. The first force is the torque generated by the motor. If you calibrate the motor for 30 inch-pounds of force, then that force will be 30 inch-pounds. The second force is generated by the momentum of the chuck. When a chuck is spinning at high speed, its rotation stores a lot of energy. This energy is determined by the speed, size and mass of the chuck. In addition to the torque generated by the motor, a second force is applied, which is generated by the momentum of the chuck. This means that a torque greater than the motor torque is applied to the cover. In this case, the total torque would be 30 inch-pounds (the torque output of the motor) plus the torque generated by the momentum of the chuck.
The chart below shows the difference in position when the cap is applied slowly and progressively according to the manufacturer's method, versus when the cap is tightened at high speed using a capping machine. When we slowly apply 30 inch pounds of torque, we tighten the cap to the proper position on the bottle, as shown below and to the left. When we calibrated the capper to 30 inch-pounds and then tightened the cap with the capper, the cap screwed tighter than if the cap had been applied slowly and gradually. This extra rotation is caused by the momentum of the chuck. The machine-tightened cap has rotated beyond its correct position on the bottle. It has been over-tightened; it exceeds the applied torque specification and therefore does not seal properly to the container.
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