Bosch 11236VS hammer drill doesn't hammer

[slantsix]

Someone gave me a Bosch 11236VS hammer drill that has lost its hammering ability. This unit has seen lots of hard pro use. Looking at the exploded diagrams, my guess is that its general means of function is that the "arm" on the "eccentric" pulls the piston rearward, compressing the main spring (pulling from the front of the spring thru linkage coming through the spring from the rear), and at a certain point in the rotation of the assembly, the piston is released, striking the rear of the striker pin, which is linked to the chuck holding the bit. The only other mechanism I can imagine is that the "eccentric arm" merely plunges the piston forward against the rear of the spring (and thru linkage to the chuck), but it seems to me that this would supply an insufficient force for the impact function. I have disassembled the unit (quite a greasy mess), and can see no broken or obviously worn parts. I could NOT see some of the forward parts which, if my memory serves me correctly -- it's been perhaps a year -- were hidden inside the impact mechanism housing, which seemed a more formidable task than I was ready to tackle without knowing more. Anyone know what part or parts are worn or broken and need replacement? (Yes, I managed to reassemble with no leftover parts, and previous drill-only functionality is restored.) Anyone know if my idea of how it functions is correct? Close?

[cmoorejack]

Hey slantsix.
the most common hammering problem with this tool is a worn o-ring on the striker..the striker moves inside the hammer piston and creates a pneumatic percussion to give more power to the hit..with a worn out o-ring and sometimes hammer piston[or chamber] the punch almost doesn't exist..also a dried out hammer piston and you have a striker that doesn't move..
try replacing the o-ring and lightly grease..[you should should have some resistence when you reinsert the striker
hope this is helpful..
Craig

[slantsix]

I never would have thought of that. Thanks very much. That really makes sense the more I think about it -- it still hammers, but with almost no effective impact. Is that a special O-ring I need to order from the supplier, or will a generic do? Am I close on how the thing functions? I.e., does the main spring's sudden release from compression provide the impact, or does it serve instead to push the striker back into the "strike-ready" position? (Or something else?)

[cmoorejack]

The o-ring is kinda special..better to purchase the correct one..1-610-210-121 ..
The area between the piston and the striker is where the air pressure is built up allowing for the extra punch..[the piston doesn't actually hit the striker..the striker hits the impact bolt that actually makes contact with the bit..!]
Craig