Rotary hammer spins but won't hammer — SDS impact mechanism diagnostics

You set the bit against a concrete wall, push in — and the rotary hammer just spins like an ordinary drill, without that distinctive pounding rumble. The bit skates across the surface and won't bite. Or the blows are still there but have gone sluggish: where a hole used to take a minute, now you lean on it for five. This is not a motor problem — the motor runs fine. The impact mechanism has failed, and that is a very different repair from a rotary hammer that won't switch on at all.

This article is about the impact chain: the piston, the striker, the SDS chuck, the mode selector and the grease inside. If your Makita HR, Bosch GBH or DeWalt drills but hammers weakly or not at all, read on. In most cases this is a recoverable fault, not a reason to scrap the tool.

Spins but won't hammer — the impact mechanism

An SDS rotary hammer has two independent drives running off one motor: rotation (the spindle turns the bit) and impact (a pneumatic mechanism delivering forward blows). That is exactly why the tool can spin perfectly and at the same time not hammer at all — rotation works, but the impact chain is broken.

The impact chain works pneumatically. The motor turns an angled gear, which moves a piston back and forth inside a cylinder. Between the piston and the striker there is an air cushion: the piston compresses air, the air throws the striker forward, the striker hits the bit shank. If any link in this chain is worn, the air cushion leaks and the blow disappears.

Common reasons the impact disappears completely:

• Worn O-rings on the piston or striker. Rubber seal rings harden and crack over time (the dry air of Riga's heating season speeds this up), the air cushion no longer forms, and the blow vanishes. This is the most common and very repairable cause.
• A worn or broken striker. The metal part that actually delivers the blow to the bit wears down or fractures over time.
• A jammed or broken piston mechanism after dry-firing into the air or after a drop.

These are all service-centre jobs. The piston and striker can't be replaced without opening the gearbox housing and the grease assembly. You won't diagnose this at home.

Weak or fading impact

A weak impact is the same story at an early stage. The blow is still there, but the energy is bleeding away. Typically it looks like this: the tool drills concrete slowly and runs hotter than before, the bit often jams and won't bite, and if you put a hand on the chuck body, you feel pronounced heat.

The main causes of a weak impact:

• Partially worn O-rings — the air cushion leaks partly, so the blow is still there but weak. This state progresses over time into a complete loss of impact.
• Dried-out or congealed grease in the impact mechanism. Old grease thickens, the piston moves against resistance, and the blow energy drops. Construction dust that has worked its way into the mechanism cakes with the grease and makes it worse still.
• The onset of striker or cylinder wear — the early stage before complete failure.

An important caveat about grease that people often confuse: the grease in the chuck neck (where you insert the bit) is a different thing from the grease deep inside the impact mechanism. The first you can and may refresh yourself. The second sits inside a sealed assembly and requires disassembly.

In practice: if the rotary hammer hammers noticeably weaker than before, do not keep working with it. A weak impact means the piston and cylinder are already running dry and wearing fast — the longer you drill, the more expensive the repair becomes. Bring the device in for diagnostics while only the air cushion has leaked, not once the cylinder is scored.

Won't switch between drill / hammer-drill / chisel

Most SDS rotary hammers have a rotary mode selector with three (some two) positions: drill only, hammer-drill, hammer only (chiselling), and often a separate chisel-angle lock as well. If the selector sticks, won't turn all the way, or pops out of position, the problem is in the mechanical selector assembly.

Typical causes:

• A worn detent. The spring-loaded ball or lug that holds the selector in position wears down, and the selector no longer takes a clean position — it slides between modes and the impact "disappears" because the mechanism isn't fully engaged.
• A broken selector or its spindle.
• Construction dust and caked grease blocking the selector's travel — it won't turn all the way.

Before you think about repair, check one thing yourself: are you turning the selector all the way at all? Many rotary hammers won't let you change mode while the spindle is still turning, or require you to nudge the chuck by hand so the teeth seat. Release the trigger fully, wait for the spindle to stop, and turn the selector to a clear click while gently turning the chuck by hand. If after that the mode still won't engage or sticks — that is already a service-centre job.

SDS bit won't stay in the chuck or falls out

The SDS chuck doesn't hold by clamping — the bit shank has longitudinal grooves, and balls inside the chuck enter those grooves, letting the bit move freely back and forth (that's the basis of the impact) but stopping it from falling out. If the bit won't stay in or drops out entirely, the chuck's retaining mechanism is worn.

Causes by symptom:

You can rule out one common and harmless cause yourself: a fouled chuck. Pull the chuck sleeve back, remove the bit, wipe the old grease and dust out of the chuck neck with a rag, insert the bit and check that the sleeve snaps back with a click and the bit is held (moving only back and forth, not falling out). Before inserting the bit, apply a thin layer of special SDS grease to the bit shank. If after cleaning the bit still falls out or the sleeve won't snap back — the ball mechanism is worn, and that is a service-centre job.

Grease, seal rings and dust ingress

The lifespan of the impact mechanism depends almost entirely on two consumable elements: the grease and the seal rings. These are exactly what Riga's construction-site conditions kill.

Concrete and brick dust is abrasive. If the seal rings are already slightly worn, dust gets sucked into the impact mechanism, cakes with the grease and starts grinding the piston and cylinder like sandpaper. Baltic humidity and condensation on building sites add corrosion to the metal parts. The dry air of the heating season, in turn, speeds up the hardening and cracking of the rubber seal rings. That is why a typical 4–6 year cycle of intensive use ends precisely with an impact-mechanism rebuild, not with motor failure.

Practical prevention that extends the life:

• Don't run the rotary hammer dry — always apply a thin layer of SDS grease to the bit shank before inserting it in the chuck.
• Don't use the rotary hammer as a chiselling tool for long stretches if it isn't designed for it — excessive chiselling speeds up striker wear.
• Avoid dry-firing into the air without a load — without back-pressure from the bit, the striker hits its stop with full energy.
• If you hear the impact going hollow or the sound changing, don't put it off — early O-ring replacement is cheap; left late, it ends with piston and cylinder replacement.

A full grease change in the impact mechanism is a service-centre job: the gearbox housing has to be opened, the old caked grease cleaned out, the seal rings and piston checked, and everything reassembled with the correct grease. For the full scope of rotary hammer repair, see our page on power tool repair in Riga.

What you can do yourself, what stays with the service centre

A clear line between what is a safe home check and what requires disassembly:

The important line: anything that requires opening the gearbox or impact housing is a service-centre job. Inside there are tensioned springs, precisely sealed assemblies and grease whose type matters — the wrong grease or incomplete reassembly quickly kills the new seal rings.

And one important distinction to rule out first: if the rotary hammer won't spin at all, won't run, smokes or sparks from the vents — that is not an impact-mechanism problem but a motor, carbon-brush or trigger-switch fault. We cover that separately in the article power tool won't turn on. This article is only about the case where the motor spins but there is no impact or it is weak.