What is the Impact Clearance in an Air Compressor?

What is the impact clearance in an air compressor?

The impact clearance refers to the distance between the top of the piston and the cylinder head of an air compressor. This clearance is a critical aspect of the compressor and should be kept as small as possible to maximize its volumetric efficiency. It should be neither too small nor too large. Furthermore, the clearance affects the efficiency of the machine and should be checked regularly.

The Importance and Impact of the Impact Clearance

In an air compressor , when the discharge valve closes at the end of the compression cycle, a small amount of high-pressure air is trapped in the impact clearance.

Before intake can resume, the air trapped in the impact clearance must expand to below the intake pressure, that is, below atmospheric pressure.

The expansion of the air trapped in the impact clearance causes stroke loss, which reduces the volumetric efficiency of the compressor. Therefore, the impact of the impact clearance can significantly affect the efficiency of the compressor.

The Impact of Low Impact Clearance

Low impact clearance can cause the piston to collide or bang with the cylinder head. This situation is particularly dangerous when the compressor is operating unloaded and the piston is moving unresisted.

Excessive impact clearance delays vacuum formation during the intake stroke, reducing the air pull required for compression. Therefore, the weight of the intake air decreases proportionally with the impact clearance size. The compressor must run longer to deliver the required compression pressure.

Reasons for Changing Clearances

During air compressor repair, if the shims installed between the cylinder head gaskets are incorrect, excessive clearance can increase, leading to bearing wear or improper bearing installation.

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How to Check Excess Clearance?

To check for excessive clearance, place a lead ball or plastic gauge on the piston and rotate the compressor by hand once.

This compresses the lead ball, and the resulting thickness is the clearance.

Use a vernier caliper or micrometer to measure this thickness and compare it to the manufacturer's value. If any deviation is found, adjustments are necessary.

Adjusting the Impact Clearance

Impact clearance can be adjusted by inserting shims (thin metal plates) into the lower bearing. Inserting shims alters the clearance between the connecting rod and piston.

What is the appropriate impact clearance?

The impact clearance typically depends on the manufacturer, but generally speaking, it should be between 0.5% and 1% of the cylinder diameter.