Short answer
Bushings are the most frequently replaced and least understood parts in a breaker. Their job is not to carry the blow but to hold the tool on axis; the shank carries the blow, the bushings only maintain the alignment. That distinction explains everything: when a bushing wears, the breaker does not stop, it loses its alignment first, and that loss starts damaging other parts. Which is why the replacement decision is made on "how much clearance" rather than "is it broken", and that question can only be answered by measurement. Judging it by rocking the tool by hand is the commonest and most expensive misjudgement.
Two bushings, two different jobs
The lower bushing is the one closest to the end of the front head and takes most of the lateral load. Being nearest to where the tool meets the material, it is also the one that wears fastest.
The upper bushing sits higher and keeps the tool on its axis. The two work as a bearing pair: when one wears, the load carried by the other rises.
On some models the two are combined into one part (a combined bushing). The function does not change, only the replacement becomes a single piece.
Why is clearance measured?
There is a designed clearance between bushing and tool shank, and that clearance is needed: for grease to enter and for the tool to move freely. It cannot be zero, but neither can it exceed a given value.
As the clearance grows the tool no longer stays on axis and moves slightly sideways on every blow. That movement makes the seal work unevenly and starts a leak; it also loads the shank and the tool retainer.
So the answer to "has the bushing finished" lies in the measurement rather than in appearance. The limit varies by model and is given in the manual; we print no figure here, because a wrong figure leads to both early and late replacement.
Rocking it by hand is not a measurement
The commonest way to assess a bushing on site is to move the tool by hand or with the boom and feel the play. That tells you something but gives no measurement: the play you feel varies with the tool length, the breaker’s position and the force applied. Measurement is taken with a gauge, with the tool removed, at the point the manual specifies. Judging by hand is only enough to say "this needs measuring".
What does the wear pattern describe?
- Symmetrical wear: the bushing has completed its normal life. The load came evenly from all directions.
- Oval wear (clearly in one direction): lateral load. The cause may be levering, dragging or working at an angle; replacing the bushing removes the symptom, not the cause.
- Fast but symmetrical wear: insufficient grease or abrasive material. Both wear in the same pattern; the site itself tells them apart.
- Scoring and marks on the inner surface: abrasive has got inside, meaning the grease barrier was not working.
- Crushing and deformation at the bushing mouth: the shank is making hard contact inside the bushing; the clearance has usually already passed its limit.
What extends bushing life?
The single most important thing governing bushing life is the continuity of the grease film on the contact surface. With the film present the two metals do not touch and wear advances only under normal loads; when the film is broken, wear accelerates and abrasive gets in at the same time.
The second factor is operator habit: every movement that produces lateral load consumes the bushing from one side. Greasing cannot compensate for that.
The third is the environment: abrasive material and dust turn the grease film into a lapping paste. On such a site the greasing interval shortens, and that shortening is set by observation rather than estimate: the grease coming out should stay clean.
What HKM uses
SolidLube: continuity of the grease film
The automatic lubrication station mounted on the breaker meters grease during the blow itself. Keeping the cushion at the bushing mouth continuous both preserves the lubricating film and works as a barrier against dust getting in; those are the two things that govern bushing life.
Look at the automatic lubrication station(solid-lube.com)Do and don’t
Do
- Take the bushing decision by measurement; get the limit from the manual.
- When changing bushings, inspect the tool shank and piston surface too.
- Look at the wear pattern; oval wear means the cause is still on site.
- Do the change in a dust free environment; the surface the new bushing seats against has to be clean.
- On abrasive sites shorten the greasing interval and watch that the grease coming out stays clean.
Don't
- Do not decide by rocking the tool and judging the play.
- Do not change only the seals and ignore the bushing; the same leak comes back.
- Do not try to partially replace a combined bushing because one side has worn.
- Do not seat a new bushing against a dusty surface.
- Do not replace an ovalled bushing and carry on without questioning the working habit.