Trying to understand how bandsaw hydraulic feed works

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05-03-2017, 11:39 AM
Nick Durkin

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Trying to understand how bandsaw hydraulic feed works

I have a bandsaw that has a hydraulic lift system where it is easy to lift up and when you set it down on your work it slowly feeds the pressure. Well it stopped doing this and is now dead weight. I tried to fill it with hydraulic fluid by filling a funnel and then lifting the bandsaw up slowly to suck in the fluid. This worked, somewhat. It is still dead weight when I lift it up, however about 3/4 of the way back down it supports itself and depending on how I have the valve adjusted, will either stay there or come down slowly. I made a diagram of the system that is on the bandsaw. The valves are basically knobs that thread in. So in the diagram they are at the bottom limit. The cylinder and both valves are shown how they are positioned on the bandsaw. If both valves are closed (at the bottom limit) the bandsaw will stop just above the work. If any of the two valves are opened slightly, the bandsaw will start to feed into the work.

I am just trying to understand what I did wrong and how at some point this bandsaw worked correctly. I am not an engineer or anything but I would like to understand the fundamentals of how this "double-action" system worked where it would help me lift the bandsaw up and stay up while I position the piece to be cut, and then when I let the bandsaw down, it would slowly feed into the work.

Thanks for the help.

Attachment 359513
05-03-2017, 11:58 AM
Mark Bolton

Id say there is some trash in the ball check that is not allowing it to seat or fully seat. The needle valve that controls the rate of feed usially has a free flow check valve incorporared so you can raise the saw head quicky. Its valved in the down direction for speed control and free flow in the other. If the ball cant seat it will just fall with no resistance.
05-03-2017, 12:44 PM
Mikail Khan

Agree. Problem is with valve. Open and clean. Compressed air may help with cleaning.

MK
05-04-2017, 2:23 PM
Nick Durkin

Thanks for the help guys. Which valve is supposed to be the check valve though? I opened up the valves and none of them have a ball inside. I can see right through. It looks like they are both flow control valves.
05-07-2017, 11:12 AM
Nick Durkin

bump bump bump
05-07-2017, 11:49 AM
Bill Dufour

There may be a check valve built into the piston. I do not understand why the circuit needs two flow control valves. maybe the extra one is for bleeding the cylinder?
Bill
05-07-2017, 9:12 PM
John K Jordan

Quote:

Originally Posted by Bill Dufour

There may be a check valve built into the piston. I do not understand why the circuit needs two flow control valves. maybe the extra one is for bleeding the cylinder?
Bill

Nick,

My horizontal saw has two valves plumbed in series, although one is a simple ball valve. One has a knob and adjusts the hydraulic bleed to control the feed rate. The simple cutoff valve locks the saw at the desired position.

My saw (JET 414459 HVBS-7MW) does not have a hydraulic lift assist. It uses a strong spring to reduce the dead weight to a manageable amount, leaving enough weight to allow descent for the cut. It is adjusted so the bow (lifting) weight is about 14 lbs.

Did you check in your manual or look for an online manual? My manual has specific instructions on the operation and adjustment for the hydraulics and bow weight.

JKJ
05-08-2017, 12:43 AM
Bill Dufour

Two valves now makes sense. One needle valve set to fine tune drop speed and another ball valve to quickly lock saw in raised position. One of those valves may have a check valve or there may be one in or on the cylinder or piston.
Bill
05-25-2017, 9:20 AM
Nick Durkin

Unfortunately I don't have a manual for this bandsaw and I cannot find a label or anything anywhere on it for a model number or name. Could someone also explain to me how this system works? I don't understand how if you have a check valve going to the piston, does it ever come down? You only have flow in one direction with a check valve right? So the cylinder gets filled up and then how does it release?
05-25-2017, 10:19 AM
Jerry Bruette

I'll assume that the cylinder rod is pushed down during the cutting stroke.

1. Oil is pushed out of cylinder into valve #1. Then free flows out of valve #1 into into valve #2, out of #2 and into the regulating portion of valve #1.
2. Valve #2 is your flow control valve. When adjusted it controls the rate of travel for your saw while in the cutting stroke.
3. When the cut is finished and you want to raise the blade oil flows out of the top of the cylinder through the free flow port of valve #1 back through valve #2 , into the lower portion of valve #1 and then returning to the cylinder.

Valve #2 would be a blocking valve, meaning it should be all the way open allowing oil to flow during the cut or completely shut to keep the saw in the up position. It should be all the way open to cut and all the way closed to hold the saw up after the cut.
Valve #1 is a a flow control valve, meaning it would regulate flow in one direction and allow full flow in the other direction. Regulate the down stroke to cut and free flow when you want to lift the blade after the cut.

You can't have a check valve in this system or the loop won't work. Oil needs to flow in both directions.

Where did you break the system to add oil?

Is it possible you removed valve #1 and turned it around when you replaced it? Then it would free flow and regulate in the wrong directions.
05-26-2017, 12:56 PM
Nick Durkin

Quote:

Originally Posted by Jerry Bruette

I'll assume that the cylinder rod is pushed down during the cutting stroke.

1. Oil is pushed out of cylinder into valve #1. Then free flows out of valve #1 into into valve #2, out of #2 and into the regulating portion of valve #1.
2. Valve #2 is your flow control valve. When adjusted it controls the rate of travel for your saw while in the cutting stroke.
3. When the cut is finished and you want to raise the blade oil flows out of the top of the cylinder through the free flow port of valve #1 back through valve #2 , into the lower portion of valve #1 and then returning to the cylinder.

Valve #2 would be a blocking valve, meaning it should be all the way open allowing oil to flow during the cut or completely shut to keep the saw in the up position. It should be all the way open to cut and all the way closed to hold the saw up after the cut.
Valve #1 is a a flow control valve, meaning it would regulate flow in one direction and allow full flow in the other direction. Regulate the down stroke to cut and free flow when you want to lift the blade after the cut.

You can't have a check valve in this system or the loop won't work. Oil needs to flow in both directions.

Where did you break the system to add oil?

Is it possible you removed valve #1 and turned it around when you replaced it? Then it would free flow and regulate in the wrong directions.

You said valve #2 is flow control valve and later said valve #1 is the flow control valve so I am not sure which one you mean.
Let me explain how the valves operate in it's current setup. When valve #1 is closed, flow is allowed from the cylinder to valve #2. If valve #1 is opened at all, valve #2 is basically bypassed because flow is allowed from all connections. So valve #2 cannot be used as a blocking valve if valve #1 is the flow valve because it will by bypassed if valve#1 is open at all and therefore with valve #2 closed, the saw will still fall. It doesn't work the other way either. If valve #1 is closed and valve #2 is opened at all, flow will still occur through valve#2 and free flow from the upper section of valve#1. Something isn't right.
05-26-2017, 3:14 PM
Doug Garson

I suggest you post a few pictured of the saw, maybe someone can recognize it and then you can find the manual for it.
05-26-2017, 9:09 PM
Bohdan Drozdowskyj

The one way check valve can be built into the piston. One valve is the speed of feed and you can set this one for zero speed, that is stopped. The other is an on/off stop valve. Make sure that the system is full of oil or it will not work correctly over the whole range of movement.
05-29-2017, 9:44 PM
Jerry Bruette

Quote:

Originally Posted by Nick Durkin

You said valve #2 is flow control valve and later said valve #1 is the flow control valve so I am not sure which one you mean.
Let me explain how the valves operate in it's current setup. When valve #1 is closed, flow is allowed from the cylinder to valve #2. If valve #1 is opened at all, valve #2 is basically bypassed because flow is allowed from all connections. So valve #2 cannot be used as a blocking valve if valve #1 is the flow valve because it will by bypassed if valve#1 is open at all and therefore with valve #2 closed, the saw will still fall. It doesn't work the other way either. If valve #1 is closed and valve #2 is opened at all, flow will still occur through valve#2 and free flow from the upper section of valve#1. Something isn't right.

OOPS!!! Sorry Nick. Didn't mean to confuse you. Any chance you could post a picture of the valves on your saw? It would help to know where you added oil into the system too.
06-07-2017, 9:55 AM
Nick Durkin

4 Attachment(s)

Attachment 361573 Attachment 361574 Attachment 361575 Attachment 361576
I am pretty sure there is a check valve built into the cylinder. When I cover the openings for the cylinder, the saw will lift, but will not go down. The valves shown are not even really valves like I am used to seeing. That is where I put the oil in though. With a funnel and then lifted the saw to suck in the oil.