Power hacksaw – powerful banging when running

[ell81Participant @ell81]

So, it’s a rapidor manchester power hacksaw.

One the end of the forward stroke, it makes a loud banging sound, like a hard thud, and is so powerful it rocks the whole saw, which as you know is very heavy.

It still cuts OK, just unacceptably loudly because of this banging.

I have tightened up the nuts on the cover for the hacksaw slides, which hasn’t stopped it.

It runs quietly when the cam is not connected to the motor and saw spindle.

But as I say, when the cam is connected with the arm that moves the hacksaw slides, there’s a huge banging sound.

Any ideas of what it could be?

All bolts are tight, I checked them.

Thanks.

[BazyleParticipant @bazyle]

CAm? what is this cam?
Anyway sounds like a worn bush.

[DiogenesParticipant @diogenes]

The bushes in the ends of the link arm or the bearings of the shaft that gives the link it’s ‘fore & aft’ motion (- the crank) are both favourite spots for wear..

 

[Clive FosterParticipant @clivefoster55965]

Had issues withe the drive pulley coming loose and walking along the shaft on mine which produced a small bang among the effects. Cured by putting the pulley back in its rightful spot and attending to the underlying fastener issue.

There seem to be considerable forces involved at reversal.

But mine is later one with a simple single pulley drive and microswitch stop. On the older ones with the mechanical clutch I’d take a careful look at the couch mechanism and components.

Clive

[halfnutParticipant @halfnut]

Connecting rod bushes and pins would be first thing I checked. Followed by the crank main bearings.

[ell81Participant @ell81]

PROBLEM SOLVED: So, I took all the bushes out, cleaned and oiled them, put them back in,tightened up the nuts on the slide, tightened up the nuts on the arm – both ends, and the big nut on the shaft for the drive pulley. One of those was causing the banging sound, I haven’t a clue which. But it is now running nice and quiet and the banging is gone.

[Howard LewisParticipant @howardlewis46836]

The forces involved at the ends of the stroke are quite substantial, even at low speed. The saw, (or connecting rod or whatever) has to be brought to a halt, and then accelerated back up to speed in a very few degrees of cam, or eccentric movement’

Near instant braking an acceleration need large forces applied for a short time!

So it pays to keep everything as free of excess play as possible.

Howard

[DiogenesParticipant @diogenes]

Never ceases to amaze me that for an item that you’d assume would be in such close proximity to ‘engineering’, how many donkey saws never seem to have seen much of the oil can..

[Diogenes]

On Diogenes Said:

(as a generalism / aside) – It never ceases to amaze me that for an item that you’d assume would be in such close proximity to ‘engineering’, how many donkey saws never seem to have seen much of the oil can..

 

[Nigel Graham 2Participant @nigelgraham2]

Howard –

Hanksawing machines, like any crank-driven machine, don’t quite behave like that.

The motion is harmonic, with the sliding parts accelerating to maximum speed at mid-stroke then decelerating to an extremely short stop at dead-centre.

The components do have considerable inertia though, so will over-run any wear until the worn walls contact with a thump.

[John HaineParticipant @johnhaine32865]

The acceleration and therefore the forces are maximum at the end of the stroke when the velocity is zero.

[Howard LewisParticipant @howardlewis46836]

Presumably, since they are machines with crankshafts, the motion in an engine is simple harmonic motion?

If so the forces to decelerate and re accelerate the con rod at each end of the stroke will be enormous at speed.  The big end will be travelling at high speed, and has to be stopped and sent back in the apposite direction in a decreasing number of crankshaft degrees.

Evidence of this were the noises emitted by steam engines (LNER and Midland for instance) where excess clearances  were closed by the acceleration/deceleration forces, and the wear on the horn guides and axle boxes that resulted.

At high rotational speeds, minimising the weights becomes important, which is why racing engines use Aluminium alloy pistons which are little more than a ring belt and gudgeon pin bosses, with minimal skirt for guidance.

Howard

 

 

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