[TheForge] Power hammer questions/Frosty

[Jerry Frost]

----- Original Message -----
From: "Bob Smolen" <boka@mwt.net>
To: <theforge@mailman.qth.net>
Sent: Wednesday, April 09, 2003 10:10 PM
Subject: Re: [TheForge] Power hammer questions/Frosty


> Frosty,
> I have a semi complete Krause type hamer. Last weekend I mounted a larger
> power cylinder and had to replumb the Krause style valve for the larger
hose
> required by the new cylinder. As an experiment before I redo the valve,  I
> thought I would try the Massey circuit and valve you described.
> I connected top to top and bottom to botom ports. I then tee'd into these
> two lines and inserted a ball valve. Just as you said, when the valve is
> open, the ram or slave cylinder does nothing. When I close off the ball
> valve, the ram rises and falls in the opposite direction of the power
> cylinder.I dont have a check valve so the ram stays on the bottom die when
> the valve is reopened.All as you described and pretty predictable.
> I should now celebrate or brag(?), but all is not well just yet.

Celebrate/brag away, what you describe below are just tuning and debugging,
not real problems. <grin>

> After a few
> good hits, the ram started to "short stroke". The ram stroke shortened and
> did not come down or go up completely.I believe this is due to the fact
that
> my balance valves -the kind described in  Mark Krause's pamphlet, are not
> large enough. I have grooves ground on the piston rods(double ended
> cylinder) at the point where when the piston is at top and bottom dead
> center, these grooves  will extend past the  seal   to allow air in or out
> of the circuit.  Because the rod is moving up and down so quickly , these
> grooves are not in position very long and I believe the "excess air" from
> the larger power cylinder binds up the slave cylinder.

> My question to you /and or the group is does this make sense?

Yes. This makes perfect sense.

> Can someone tell me the approx.
> size of groove needed? My power cyl. is 5" dia and 6 in. stroke. The slave
> is 2.5 in. dia. and 10 in. stroke. The displacement ratio is about 2/1, I
> believe.The gear reduced speed is about 275 strokes per min.

I have no idea how much groove you need. However, in the few drawings I've
seen nobody used grooves for balance valves/ports. Most have flat sections
in the piston rods where you have grooves, they look like wrench flats. If
you don't think you can make the grooves larger make them longer so they
stay open longer, at 2:1 you have more than enough air.

Or you might be able to grind flat sections in the rods you have. You should
be able to make the depth of the flats about 10% of the rod diameter (each)
without weakening the rod appreciably. For example you could grind 1/8" deep
flat sections on opposite sides of a 1 1/4" diameter rod and not weaken it
appreciably.

Shortening the stroke on your compressor piston so your ratio is closer to
1:1 might make the problem go away altogether.

> I tried  to follow your response to Bob Schade below. If the ram cylinder
is
> mounted so that  the cylinder cannot fully extend before the top die meets
> the bottom die, should you be concerned about knocking out the bottom end
of
> the cylinder?

What I was trying to say is the ram is controlled totally by the compressor
piston so it should just be a matter of stroke length to keep the ram piston
from bottoming (topping?) out on the return stroke. The ram piston hitting
the end of the cylinder WILL eventually hammer it to pieces.

Massey extended the ram cylinder bore into the head, past the top port so
there was a dead space. When the piston entered the dead space on the up
stroke the air trapped there compressed and cushioned it. When the
compressor piston reversed direction (slightly before the ram piston reached
the top) it put the bottom of the ram piston under a vacuum. The vacuum,
coupled with the compressed air in the dead space not only stopped the ram
piston cold but reversed it quickly.

> I believe it is a good idea, as you said, to have more rather than an
> inadequate amount of displacemnet . If this is done, you should be sure
the
> balance valves are of adequate size.
>

I must've been unclear. It's not surprising, this is a difficult way to talk
about machinery. I MUCH prefer face to face, coffee, pencil, paper and maybe
the actual machinery.

What I was trying to say is: oversizing the pistons is better than
undersizing or even guessing. The displacements MUST be balanced properly
though.

If you're pushing a conbined 100 lbs of ram weight and a commercial hammer
uses a 4.5" dia ram piston pushed by a 6" compressor piston. (I'm using
piston rods of 1.25" dia. for the purpose of discussion only) Diameters
alone have a ratio of 1.84:1 compressor to ram. I don't know what the
compressor piston stroke is but assume it yields a displacement close to the
1.2:1 - 1:1 I've inferred from drawings. This hammer also produces pressure
peaks of 15 psi.

So, from the example we know a 4.5" ram piston under 15 psi is enough to
operate a 100 lb. hammer. Assuming a 1.25" rod, the ram piston has 14.7
sq/in area. At 15 psi. this is 220 lbs. of force.

The compressor displacement can be adjusted by changing it's stroke length
so for the purpose of discussion let's just assume it's balanced to the ram.
In what I call an oversized system, the ram piston(s) have larger area(s)
but balanced displacements. This means lower operating pressure or greater
ram force.

For example a 6" ram piston with a 1.25" rod have an area of 27 sq/in. To
produce 220 lbs of force it needs 8.2 psi. Or at 15 psi. developes 406 lbs.
of force.

Looking at the example of the commercial hammer though we don't see the 4 or
5:1 force to weight ratio. This is because I've ignored the effects of the
vacuum acting on the opposite side of the ram piston. I've left it out
simply because I have no idea what kind of partial vacuum to expect but for
my own guestimating I've been using -10psi.

> I did not try using holes at the equator of the ram cylinder for balancing
> the system(ala Massey) but I like that idea as well because I believe you
> can move more air that way without making grooves in the cylinder rod
which
> could weaken the rod.
> I really appreciate your info on Masseys. This valve  circuitry may really
> simplify the home built self contained  hammer.
> As a post script to my speculation that the balance valve son my hammer
are
> inadequate, I did enlarge the grooves and the hammer seemed to be working
> much better . I only ran it a short while before one of my welds broke, so
I
> dont want to say I am sure this is the fix. After some repairs this
weekend,
> I hope to give it another try.
> Thanks for any comments. Do we have a fluid power technician in the group?
> Regards,
> Bob S.
>
>

Thanks for the report, it made my day. <grin>

Frosty
------------------------
If it ain't forged
it ain't real.
Wrought iron is.
The FrostWorks

Meadow Lakes, AK.