Clearance is necessary in many mechanical joints. Uncontrolled clearance is not. On heavy equipment, visible axial movement in pins, cylinders and linkage joints is often treated as a minor issue because the machine can still perform its basic function. In practice, that movement can be the beginning of a much faster wear process.
Axial movement changes how the joint is loaded
A pin-and-bushing joint is designed to carry load through defined contact surfaces. When excessive side movement is allowed, components no longer remain consistently aligned. The load can move toward the edges of the bushing, pin eye or bearing surface instead of being distributed as intended.
That produces several effects at the same time:
- higher local contact pressure;
- edge loading;
- impact when the direction of force changes;
- loss of lubrication film at the most heavily loaded points;
- progressive deformation and wear of the mating surfaces.
The important point is that axial clearance does not remain only axial. Once the contact surfaces begin to wear unevenly, radial clearance starts to increase as well.
From movement to impact
Consider a boom cylinder or Z-bar linkage. During loading, lifting, lowering or reversing direction, forces repeatedly change direction. If the joint can move sideways before the load is taken up, the components do not simply transfer force — they first move and then strike the opposite contact surface.
A small amount of movement therefore becomes a repeated impact cycle.
The operator experiences this as knocking, looseness or a machine that does not feel tight. Mechanically, the joint is experiencing dynamic loading that can be much more damaging than a stable static load.
Why shims matter
Correctly selected shims are a simple way of controlling axial position while maintaining the clearance required for free movement. They help keep cylinders, links and pins in their intended position and reduce the distance through which components can accelerate before contact occurs.
Shimming is not a substitute for correct manufacturing tolerances. It is part of a controlled assembly concept when the design requires adjustment of axial clearance.
The objective is not to remove every micrometre of movement. The objective is to achieve controlled clearance without binding throughout the full range of motion and under realistic operating temperatures.
New machines create a customer expectation
There is also a commercial dimension. A customer purchasing a new heavy machine expects it to feel new: joints should be controlled, structures should not knock unnecessarily and the machine should give an impression of precision.
Visible or audible looseness on a new machine immediately raises questions about assembly quality, even when the movement has not yet caused a functional failure.
This matters because product quality is judged not only by whether a machine can perform its rated task. It is also judged by how the machine behaves while performing it.
A better approach
For joints where axial position is adjustable, the better practice is to define:
- the acceptable axial clearance range;
- the available shim thicknesses;
- the measurement method during assembly or pre-delivery inspection;
- the minimum clearance required to prevent binding;
- the inspection interval for high-load joints.
When distributors or service teams repeatedly need to correct excessive axial clearance on new equipment before delivery, that is useful feedback for the manufacturer. The most efficient solution is to control the condition during production, not to treat each machine individually in the field.
The principle
The engineering principle is straightforward: movement changes alignment, misalignment concentrates load, concentrated load accelerates wear, and wear creates even more movement.
Breaking that cycle early is inexpensive. Correcting it after pins, bushings and bores have worn is not.