A pivot bushing on an excavator arm is replaced during a scheduled service. Two weeks later it has walked halfway out of its housing and the pin is hammering against the bore. The new bushing measured perfectly. So did the pin. Nobody measured the housing, which had worn oval from years of movement. The bushing was pressed into a hole that could no longer grip it. Getting the bushing interference fit right starts with the hole, not the part.
What an Interference Fit Does
In an interference fit (also called a press fit), the outside diameter of the bushing is slightly larger than the bore of the housing. When it is pressed in, the bushing is squeezed and the housing is stretched. The friction created by that pressure is what stops the bushing from rotating or walking out. No adhesive, no set screw, just geometry.
Too little interference, and the bushing loosens under load or vibration. Too much, and you risk cracking the housing, galling the surfaces during installation, or collapsing the bushing’s inner bore so far that the pin no longer fits.
Bore Closure: The Detail That Catches People Out
When a bushing is pressed in, its inside diameter shrinks. This is called bore closure. A thin-walled bushing in a stiff steel housing can lose most of the interference value from its inside diameter. That means a bushing machined to the correct inner size before installation can end up too tight on the pin afterward.
There are two ways to handle this:
- Machine the bushing’s inner bore oversize by the expected closure, so it lands on size after pressing
- Press the bushing in first, then finish the inner bore in place (line boring or reaming), which gives the most accurate result for critical pivots
Choosing the Right Amount of Interference
| Factor | Effect on interference choice |
|---|---|
| Housing material | Cast iron and aluminum need less interference than steel to avoid cracking |
| Bushing material | Bronze and polymer bushings behave differently from steel under pressure |
| Wall thickness | Thin walls collapse more; thick walls transfer more stress to the housing |
| Operating temperature | Different expansion rates can loosen or tighten the fit when hot |
| Load type | Shock and oscillating loads need more grip than steady loads |
Standard fit systems such as ISO 286 give tolerance classes for shaft and hole combinations. They are a good starting point, but the final choice should consider the factors above, not just a table value.
Installation Matters as Much as Dimensions
- Clean both surfaces; any burr or debris creates uneven stress
- Use a lead-in chamfer on the bushing and the housing
- Press straight using a proper mandrel; never hammer at an angle
- For heavy interference, shrink the bushing by cooling it or expand the housing by gentle heating, instead of forcing it
- Measure the inner bore after installation, every time
When the Housing Is the Problem
A worn or oval housing cannot hold any bushing reliably. The repair is to machine the housing round and oversize, then make a bushing with a matching larger outside diameter. This is where a custom bushing beats a catalog one: it is sized to the real hole, not the original drawing.
How Merdeka Precision Makes Bushings
Bushings, shafts, and machine mechanical components are core items in our fabrication service. We work from your drawing or from measurements of the actual housing and pin, so the interference is calculated from real dimensions. Parts are machined on our lathes and milling machines, finished by grinding where needed, and verified with inside and outside micrometers and inside dial calipers during QC inspection.
Because bushings are often needed in specific oversize dimensions and small quantities, they are usually produced as job orders. We regularly supply customers whose equipment works in tough conditions, including the construction industry, where pivots take shock loads every day.
Measure Three Things
Before ordering a replacement bushing, measure the housing bore, the pin diameter, and the old bushing. Record each at several angles. If the housing is out of round, fix that first.
When a bushing on your equipment wore out last, did anyone check the housing it came out of? Send us those measurements and we will help you size the replacement correctly.