Sawmill parallel cylinder lift issues

[Rorancev]

Hi, not sure if I’m posting in the right place. I have ran into a problem on my sawmill that I have designed and fabricated. The issue is in the lift mechanism, what I like to call the “carriage” it is a constant uneven load it is a bandsaw/resaw sawmill that slides on a forty foot trailer. The uneven load is due to one side holding the motor and most of my hydraulic motors/ controllers while the other side is bare with minimal weight compared to the opposing side. I’d estimate the difference is 300 pounds to 150 pounds. Out of the pump I go into my directional controller and then out of the controller to lift I run into a flow divider for my cylinders. The carriage does actually lift quite evenly within 1/8 of an inch difference on each side and I am satisfied with that. The problem is after I have chose my desired height to cut at, the cylinder with the heavier load on top gradually weeps down while my cylinder with less load gradually weeps up creating a bind on my roller mechanism which is some steel tubing with rollers on it. Also my down feed is annoyingly different the heavier side goes faster. I’m curious what someone would do to remedy these issues I have considered buying another flow divider for the feed down and after the flow divider on each “T” running those into a double pilot operated check valve to lock the line essentially. It’s quite an investment for those 3 components and I’m just hesitant because I don’t want to be out that money and have the problem still persist. I’ll get some pics so you guys have a visual understanding

 

[Doug Hunter]

Nice work on your machine! It sounds like you're looking at the right things, but it's hard to diagnose specific problems from afar without the details of the hydraulic circuitry. It definitely sounds like a leak from the high-load cylinder to the low-load cylinder. Can you provide a hydraulic diagram of the system from the directional controller to the cylinders? It would also help to see a diagram of how the cylinders are mechanically lifting and holding the carriage. The key may be in the design of the lift mechanism to make sure it mechanically stays parallel to the trailer (if I'm understanding you correctly).

 

[PNachtwey]

I understand. I am very familiar with big sawmills where horizontal band saws the moved up and down user servo qualify valves and have MDT feedback. How does Wood Mizer do this? I am surprised a geared flow divider isn't more accurate going up. Ditto the look for leaks. I find the obvious ones by listening for them or feeling for the heat they generate.

 

[FluidPowerUser]

The issue here is the load differential across the cylinders. The flow divider will do its best to compensate for the pressure differential, but one leg of the flow divider will see higher leakage around the gears and it may also be spilling oil across the relief valve and that is why the accuracy is not quite as good as it could / should be.

Under static load, with one cylinder seeing a high pressure, even with the flow divider, the second law of thermodynamics will apply. High pressure will find it way to low pressure, until an equilibrium is established. One cylinder will go down and the other one will go up. The is simply because the oil leaks from the highest pressure region, across the flow divider and into the low pressure cylinder. It will only stop when the pressures are equal.

You will need to put a counterbalance valve in the lift side of the cylinders. Then add pilot lines to open them to lower the cylinders.