FLOW CONTROL ON A LOG LIFT
There are a variety of ways to control the speed of a cylinder. I am not saying a log lift needs to be this complicated. 90% of simple homemade projects work just fine by throwing stuff together. The solution could be pretty crude and work just fine, but for those who want to understand more or apply to other applications, here are some factors to consider.
1. Restricted spool in the valve. Requires a different spool which is not real desirable or maybe not available.
2. Limit the motion of the existing spool. If the spool moves 1/4 inch for full travel, can you limit it to moving 1/16 or 1/8 inch?
Bunny trail: Two things happen inside an open center valve when you move the spool: the cylinder is gradually opened to the P connection, and the connection from P to the outlet (T) is gradually closed off. (There is a 'load check' on most open center valve sections, but a log splitter valve might not have one.) This causes the lift cylinder to move. It may not be a linear effect, you may have to move the spool half of its travel to get 1/4 flow or something.
Anyway, some ways to limit stroke, it depends on the valve design which if any would work, but I have done them all at various times:
-Drilling and tapping the end caps and putting in a bolt and jam nut to hit the end of spool. Screwing it in or out changes how far the spool can move.
-Putting shims on the spring stack end (far end from the handle) and altering how the spool moves. Usually the stroke of the spool is limited by this mechanism, not by handle motion or any external device. The same spring and spacer control spool motion in both directions, so it takes some creativity.
-Make a flat iron bracket that the handle hits so it can only go part stroke.
3. Flow controls
They can be either fixed or adjustable.
They can be in the line with the same orifice metering in both directions
Most commonly, they have an adjustable orifice, with a built in check valve so it meters in one direction but free flows around the orifice in the other direction.
Some are just a check valve with a hole drilled through the center, or a floating disc with hole through the middle, or a check with some grooves cut in the seat to make the check leak in the backwards direction. I think Surplus Center had some of the old Gresen restrictors of that type. Those are good and simple, but not adjustable.
They can be installed restricting the flow going toward the cylinder (meter into load) or the flow coming out of cylinder (meter out). For a load that is always one way and always resistive (opposing the motion, like the log lift), meter in would work fine. For a load that can go into 'overrunning' (wanting to run ahead in the direction it is being pushed), like a vehicle ground drive, a vertical hanging load in lowering, or a lever arm that can go over center, the control has to be meter out. The load has to be pushing the oil against the flow control restriction in 'compression' to maintain control. Putting oil into pulling 'tension' doesn't work so well : )
Meter-in creates the pressure drop before the fluid reaches the load. Load pressure will be always less than the pump maximum pressure. The outlet side of the load will be at a low pressure as there is minimal restriction there.
Meter out maintains control but creates a high fluid pressure on both sides of the motor or cylinder. In some situations, most commonly a vertical load hanging from the cylinder and tending to pull the rod out/down, a meter out flow control can create very high pressures on the cylinder. Due to the area difference of the piston, the rod side pressure will be the weight of the load plus the full pump pressure intensified by the cylinder area ratio, pushing the piston down. So a cylinder with a 1000 psi load pulling on the rod side, and 2500 psi on the closed side from the pump, could easily reach 4500 psi intensified pressure between the cylinder and the flow control. Obviously this could blow rod seals or damage the steel tube. Intensification normally only occurs when the main control valve is shifted fully and all control is being done by the flow controls. Intensification is no unusual deal, just a normal situation the designer needs to check with certain types of loads.
Counterbalance valves, also called over center or load control valves, are used in those situations to maintain control but not intensify the pressure. They also control the load if a hose breaks. I won't go there now.
4. Flow dividers (spool type)
Proportional dividers split a flow into two parts proportionally: 50/50, or 75/25 for instance. As inlet flow varies, the outlets vary but stay in proportion to each other.
Priority dividers put a constant flow out the priority port, and the rest out the bypass. For example, power steering pumps have a pf divider to send a constant flow to steering while the engine rpm and pump flow vary a lot. Without this, power steering could get exciting at high engine speeds.
However, both types of spool divider works by throttling the unused side to force oil out the priority port against a load. So if the inlet is 10 gpm, and the priority load is 1 gpm at 1000 psi, the other 9 gpm going out bypass will be throttled to create 1000 psi load. This 9 gpm at 1000 psi in converted entirely to heat. No big deal on intermittent circuits, but it takes some thought before using them on continuous flow conveyor motors for instance.
A gear type divider could work well, but that is another topic. More complex than I think you need.
5. Misc notes
You want the restriction or controlling device between valve and cylinder, not on the P or T line into the main directional valve.
Putting any sort of valve in series downstream of the original splitter valve most likely defeats the relief valve in the original valve. You will need either a RV either built into the new valve, or an external one not inside either direction valve. The original relief valve will have high pressure on both upstream and downstream and has nowhere to relieve to. That is one of the reasons for power beyond (third port) valves. There are other reasons also.
If you have a two section original directional valve, this is already taken care of internally in that valve.
All these factors effect the designers choices and decisions. This is a very simple case. For a log lifter, I assume the load is on the closed side, rod extend to lift, gravity load always resisting extend or assisting retract.
Unless your platform can go past center, you don't need to worry about overrunning load on the rod side. Thus, in the raising direction you could control either meter into the closed side or meter out of the rod side.
In the lowering direction, the load is overrunning so you have to meter out of the closed side to control the lowering speed. Intensification is less likely in the rod retract direction because the area ratio is less than 1, but I would check it anyway just out of habit. I am guessing without having your load or cylinder sizes, but with 2500 psi on the rod side, you probably get 1500-2000 created on the closed side from intensification. (It is actually a reduction in pressure by the cylinder area ratio in this direction.) In the lowering direction with no load you should be well under the maximum rating for most cylinders and seals. Lowering with a heavy load could create high pressures if the valve was yanked full stroke. Granted, if the operator 'feathers' the spool valve properly it will not be an issue. However, if the operator is good, none of the controls are necessary and we would not be addressing it. The whole point of flow control is to protect against operator error, so we have to assume the operator has yanked the lever to full stop.
So, assuming you have checked for intensification on the lowering direction, here are some suggestions:
1. Run small lines (¼ inch) to the cylinder. This is adding restriction.
2. Limit the spool stroke of the directional valve as noted above. This controls both meter in and out and would be the easiest.
3. Putting the flow control right at the cylinder can help protect against loss of control if a hose breaks, but that depends on your spaced layout.
4. A fitting welded up and drilled, or tapped for a small pipe plug that can be drilled, right at the cylinder closed end port, would be fixed and in the circuit both directions. Easy, cheap, non-tamperable, but since the load is different in each direction, this might take some fiddling. But it is easy to try.
5. A simple needle valve would be adjustable, but still in the circuit both directions. Same issues as number 4.
6. Two inline adjustable flow controls with free flow reverse checks.
Install one of them metering out of the closed side of cylinder to control lowering speed.
To control raising speed, the second flow control could be installed metering out of the rod side. Or, it could be installed in series with the first fc on the closed side (before or after) in the meter in direction.
The fixed orifice drilled fitting is worth a try on the cheap. Two flow controls would be the most commonly done. I would use ¼ inch lines, and spend the money and add the two FC.
kcj
There are a variety of ways to control the speed of a cylinder. I am not saying a log lift needs to be this complicated. 90% of simple homemade projects work just fine by throwing stuff together. The solution could be pretty crude and work just fine, but for those who want to understand more or apply to other applications, here are some factors to consider.
1. Restricted spool in the valve. Requires a different spool which is not real desirable or maybe not available.
2. Limit the motion of the existing spool. If the spool moves 1/4 inch for full travel, can you limit it to moving 1/16 or 1/8 inch?
Bunny trail: Two things happen inside an open center valve when you move the spool: the cylinder is gradually opened to the P connection, and the connection from P to the outlet (T) is gradually closed off. (There is a 'load check' on most open center valve sections, but a log splitter valve might not have one.) This causes the lift cylinder to move. It may not be a linear effect, you may have to move the spool half of its travel to get 1/4 flow or something.
Anyway, some ways to limit stroke, it depends on the valve design which if any would work, but I have done them all at various times:
-Drilling and tapping the end caps and putting in a bolt and jam nut to hit the end of spool. Screwing it in or out changes how far the spool can move.
-Putting shims on the spring stack end (far end from the handle) and altering how the spool moves. Usually the stroke of the spool is limited by this mechanism, not by handle motion or any external device. The same spring and spacer control spool motion in both directions, so it takes some creativity.
-Make a flat iron bracket that the handle hits so it can only go part stroke.
3. Flow controls
They can be either fixed or adjustable.
They can be in the line with the same orifice metering in both directions
Most commonly, they have an adjustable orifice, with a built in check valve so it meters in one direction but free flows around the orifice in the other direction.
Some are just a check valve with a hole drilled through the center, or a floating disc with hole through the middle, or a check with some grooves cut in the seat to make the check leak in the backwards direction. I think Surplus Center had some of the old Gresen restrictors of that type. Those are good and simple, but not adjustable.
They can be installed restricting the flow going toward the cylinder (meter into load) or the flow coming out of cylinder (meter out). For a load that is always one way and always resistive (opposing the motion, like the log lift), meter in would work fine. For a load that can go into 'overrunning' (wanting to run ahead in the direction it is being pushed), like a vehicle ground drive, a vertical hanging load in lowering, or a lever arm that can go over center, the control has to be meter out. The load has to be pushing the oil against the flow control restriction in 'compression' to maintain control. Putting oil into pulling 'tension' doesn't work so well : )
Meter-in creates the pressure drop before the fluid reaches the load. Load pressure will be always less than the pump maximum pressure. The outlet side of the load will be at a low pressure as there is minimal restriction there.
Meter out maintains control but creates a high fluid pressure on both sides of the motor or cylinder. In some situations, most commonly a vertical load hanging from the cylinder and tending to pull the rod out/down, a meter out flow control can create very high pressures on the cylinder. Due to the area difference of the piston, the rod side pressure will be the weight of the load plus the full pump pressure intensified by the cylinder area ratio, pushing the piston down. So a cylinder with a 1000 psi load pulling on the rod side, and 2500 psi on the closed side from the pump, could easily reach 4500 psi intensified pressure between the cylinder and the flow control. Obviously this could blow rod seals or damage the steel tube. Intensification normally only occurs when the main control valve is shifted fully and all control is being done by the flow controls. Intensification is no unusual deal, just a normal situation the designer needs to check with certain types of loads.
Counterbalance valves, also called over center or load control valves, are used in those situations to maintain control but not intensify the pressure. They also control the load if a hose breaks. I won't go there now.
4. Flow dividers (spool type)
Proportional dividers split a flow into two parts proportionally: 50/50, or 75/25 for instance. As inlet flow varies, the outlets vary but stay in proportion to each other.
Priority dividers put a constant flow out the priority port, and the rest out the bypass. For example, power steering pumps have a pf divider to send a constant flow to steering while the engine rpm and pump flow vary a lot. Without this, power steering could get exciting at high engine speeds.
However, both types of spool divider works by throttling the unused side to force oil out the priority port against a load. So if the inlet is 10 gpm, and the priority load is 1 gpm at 1000 psi, the other 9 gpm going out bypass will be throttled to create 1000 psi load. This 9 gpm at 1000 psi in converted entirely to heat. No big deal on intermittent circuits, but it takes some thought before using them on continuous flow conveyor motors for instance.
A gear type divider could work well, but that is another topic. More complex than I think you need.
5. Misc notes
You want the restriction or controlling device between valve and cylinder, not on the P or T line into the main directional valve.
Putting any sort of valve in series downstream of the original splitter valve most likely defeats the relief valve in the original valve. You will need either a RV either built into the new valve, or an external one not inside either direction valve. The original relief valve will have high pressure on both upstream and downstream and has nowhere to relieve to. That is one of the reasons for power beyond (third port) valves. There are other reasons also.
If you have a two section original directional valve, this is already taken care of internally in that valve.
All these factors effect the designers choices and decisions. This is a very simple case. For a log lifter, I assume the load is on the closed side, rod extend to lift, gravity load always resisting extend or assisting retract.
Unless your platform can go past center, you don't need to worry about overrunning load on the rod side. Thus, in the raising direction you could control either meter into the closed side or meter out of the rod side.
In the lowering direction, the load is overrunning so you have to meter out of the closed side to control the lowering speed. Intensification is less likely in the rod retract direction because the area ratio is less than 1, but I would check it anyway just out of habit. I am guessing without having your load or cylinder sizes, but with 2500 psi on the rod side, you probably get 1500-2000 created on the closed side from intensification. (It is actually a reduction in pressure by the cylinder area ratio in this direction.) In the lowering direction with no load you should be well under the maximum rating for most cylinders and seals. Lowering with a heavy load could create high pressures if the valve was yanked full stroke. Granted, if the operator 'feathers' the spool valve properly it will not be an issue. However, if the operator is good, none of the controls are necessary and we would not be addressing it. The whole point of flow control is to protect against operator error, so we have to assume the operator has yanked the lever to full stop.
So, assuming you have checked for intensification on the lowering direction, here are some suggestions:
1. Run small lines (¼ inch) to the cylinder. This is adding restriction.
2. Limit the spool stroke of the directional valve as noted above. This controls both meter in and out and would be the easiest.
3. Putting the flow control right at the cylinder can help protect against loss of control if a hose breaks, but that depends on your spaced layout.
4. A fitting welded up and drilled, or tapped for a small pipe plug that can be drilled, right at the cylinder closed end port, would be fixed and in the circuit both directions. Easy, cheap, non-tamperable, but since the load is different in each direction, this might take some fiddling. But it is easy to try.
5. A simple needle valve would be adjustable, but still in the circuit both directions. Same issues as number 4.
6. Two inline adjustable flow controls with free flow reverse checks.
Install one of them metering out of the closed side of cylinder to control lowering speed.
To control raising speed, the second flow control could be installed metering out of the rod side. Or, it could be installed in series with the first fc on the closed side (before or after) in the meter in direction.
The fixed orifice drilled fitting is worth a try on the cheap. Two flow controls would be the most commonly done. I would use ¼ inch lines, and spend the money and add the two FC.
kcj
