How Do Air Straightness Plugs Work?

Air straightness plugs are slightly more sophisticated than conventional air plugs, but still have all of the benefits of a normal air plug: they are simple to set up and operate, and they produce highly accurate results.


A conventional air plug includes four measurement jets, two in the center and two at the ends, arranged in two groups. The plug’s structure enables it to see both ends of the bow situation. The exact location of the jets in relation to one other is not governed by any standards, as is the situation with squareness or taper tests. There are no ratios concerned, either.

The air nozzles at the plug’s extremes are designed to check for non-straightness, which is normally defined for the bore‘s whole length. However, before we can grasp the way in which a straightness plug functions, we must first look at the different combinations of jets that are common in air tooling.

Differential Measuring

A differential measurement system is what is associated with a two-jet plug. Picture a two-jet gas plug with a zero readout within a master ring. Adjust the plug such that one of the jets is positioned on the ring’s side. This raises posterior force on one jet while lowering it for the other.

A four-jet system is an expansion of the two-jet gas plug. Four jets are combined together in this case, and in the event that the plug is shifted in any manner, an aggregate reading is taken once again. The four jets each detect four changes in pressure and sum them up. The total—and the readout on the indicator—changes whenever any of the recorded dimensions fluctuates.

On the plug, the 4 jets are usually at equivalent levels or planes. The 4 jets may theoretically be moved individually anyplace along the plug, and in the event that they are situated at ninety degrees relative to each other, they would measure the bore’s mean diameter. The 2 jets on top are counterbalanced by the 2 jets on the bottom, resulting in no change in the show. The aggregate pressure fluctuates if the bore is not completely straight, and the differential would be displayed on the instrument.

Dynamic Measuring

The display gives a number in the event that the straightness plug is merely put into the bore. The key question is what that figure implies. Whenever the jets are aligned with the bow, according to the orientation, they obtain their maximum or minimum reading. When the plug is rotated one hundred and eighty degrees, the outer and inner jets switch roles and show the same value, indicating that the plug is in a differential state.

However, when the plug rotates one hundred and eighty degrees to explore the bore, the sets of jets would have a peak clearance, followed by a minimum clearance, generally at ninety-degree angles to one another. As viewed along the whole extent of the plug measuring length, the difference between the greatest and minimum value would be the out-of-straightness state.

by George Chitos