Cordless impact driver - Where does the power come from?

Discussion in 'Tool Talk' started by Roy66, Jan 30, 2013.

  1. Roy66

    Roy66 New Member

    I've heard a lot of great things about cordless impact drivers. It seems that every article I read on them mentions that they have a longer battery life than regular cordless drills because they don't use any additional battery power to create those powerful impacts.
    How can this be true? Surely that power must be coming from somewhere?
  2. Fat Tony

    Fat Tony New Member

    They have a mechanical hammer mechanism built in. They are superb for driving large screws into timber, too brutal for delicate work such as kitchen cabinets.
  3. Roy66

    Roy66 New Member

    But surely that mechanism has to be powered by the motor/battery? How is it possible that these tools don't use additional power?
  4. Fat Tony

    Fat Tony New Member

    It just spins.....
  5. ibanezman

    ibanezman Member

    I'd always just assumed that as it spins like a normal drill, it hits some sort of solid lug which causes it to have that hammer action.
  6. ibanezman

    ibanezman Member

    Looks like my guess was right:

    There's nothing to 'power' the hammer action - it's just the same, but the spinning bit is spinning over a bumpy surface, as opposed to a smooth one, causing the hammer effect.
  7. Roy66

    Roy66 New Member

    Thanks for your reply, but the link you've posted discusses impact wrenches. My question is about impact drivers, which work differently. An impact driver produces continuous torque exactly like a cordless drill, but when it meets resistance an additional torque is produced by a hammer and anvil mechanism pounding the side of the bit. Reviews of impact drivers claim that the hammer and anvil mechanism doesnt require battery power, and so the tool produces twice the torque without draining the battery. I'm wondering how this is possible.
  8. ibanezman

    ibanezman Member

    Oh I see, I hadn't quite grasped the point of the question. This video might give you an insight:

    I wonder if it works by 'storing' energy. So, for example, if the mechanism was sprung in some way, the spring would compress as it hit resistance and use the force of the spring trying to expand to create additional torque. I'm guessing of course, but it's the same principle as they use in racing cars to turn the braking energy into forward motion.
  9. Fat Tony

    Fat Tony New Member

  10. ibanezman

    ibanezman Member

    Found an answer - it's geared it seems, choosing a hgiher gear when it needs more torque.

  11. Roy66

    Roy66 New Member

    That Youtube video was really interesting, and your point about the gearing makes sense! I can see how it works now.
  12. ibanezman

    ibanezman Member

    You're welcome. I like to know how things work, so it was good to find out!
  13. Hmm, gearing certainly does alter the torque, but not in the way described above. If you want more torque, then you need a lower gear - like knocking down a gear when going up a steep hill.

    That x-ray was fascinating, tho'. It does seem to differ from a normal hammer drill - the mechanism appears to be reversed! In a normal hammer drill, the chuck is effectively connected to a round disc that has ridges in it. A non-rotating ridged disc in contact behind this disc makes the chuck vibrate back and forth rapidly, but a very small amount.

    In the impact driver shown in the x-ray, there is a similar (and, I think, non-rotating) disc sitting on the chuck shaft, but its surface profile - instead of regular up/down ridges - is that of a 'drop-cam', ie: a smooth upwards slope for half the disc followed by a sharp drop. This heavy disc can slide on the chuck shaft and is held forward by a spring. In front of this is the mechanism which effectively acts as the cam 'follower' and has a point on it which engages with the cam profile mentioned above. As the follower turns, it pushes on this slope and moves the disc backwards until it reaches the peak of the profile, at which point it now 'drops' forward under the force of the spring. This gives a slower, but heavier, forward hammering impact.

    So, instead of the steady, smooth, 'brrrrrrr' of the hammer drill, the impact drill gives much fewer, but heftier, 'punches'. (If you used a normal powered screwdriver and gave the back a few regular hammer blows, you'd have the same effect...)

    HOWEVER, more 'torque'? I think not - torque is a completely different kettle of worms; it's the turning force. And that isn't being increased by the hammering action.

    I guess, by 'weakening' the timber by the hammer blows, the wood is effectively made easier to drill in to, and this means that the drill can maintain its torque level, but it doesn't - as far as I can see - actually increase the drill's torque output. I think that's a distortion of the facts.

    Want me to talk some more, or should I shut up?

    Ok, then... :'(
  14. snezza31

    snezza31 New Member

    As Fat Tony said, just try one! You will be pleasantly surprised how easy it will drive a screw in compared with a normal battery drill/driver.

    However,I have to disagree with him regarding his statement about an impact driver and the fitting and fixing of kitchen cabinets. I use my 14volt Bosch Impact EVERY DAY for fixing cabinets to the wall and for fixing them to each other. The variable speed is really subtle and the impact clutch works a treat.
    When you are stetching at arms length in the back of a cabinet, it is sometimes difficult to push with any major degree of pressure to drive a screw in at a strange angle. With the impact driver, it is practically effortless.

  15. joinerjohn1

    joinerjohn1 Screwfix Select

    +1 for snezza's post above. An absolute must have tool for kitchen fitters. ;)

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