Gravity and lowering hydraulics 4

[hydroman247]

Hi everyone.

I have a question for you. Do any big manufacturers of machinery use gravity to lower hydraulic booms? Obviously most are powered down using counterbalance valves which uses energy and in this day and age, efficiency is more important than ever.

I can't find anything online but have been told that JCB have some sort of gravity assisted system.
Is there anything else I should know about these systems?

Thanks

 

[hydroman247]

All the controls are fully proportional at the moment. The operator has full control over the speed (withing certain parameters set by various safety standards).

The accidental joystick thing should not be an issue. We have to use a footswitch before any of the controls are active. All this runs off a CAN system with a PLC so we can go either way in regards to software or hardware.

Your cylinder mounted design is interesting. The only issue I can see right now is like you say, a possible slight jolt when energising the on/off solenoid but I am sure I can think of a way around that. If there is any failure in the system, the cylinder moving in any direction won't be acceptable. This is why counterbalances and O/C valves work so well in this situation.

As for the manual hand pump lowering, this is the awkward one and would need yet another valve closer to the cylinder than the electronic valves in case of power loss.

 

[hydtools]

Don't forget duplicate controls at the base for use if the bucket rider is incapacitated. Or radio-controlled valve system in the event the bucket and truck are electrified.

Ted

 

[hydtools]

Another thought. If this machine is to be used near energized power lines, I doubt you will be permitted to run wires up to the bucket. Non-conductive booms require the use of non-conductive control system to the bucket.

Ted

 

[Oldhydroman]

If you can install the proportional throttle valve very close to the on/off valve then the volume of oil between the two components will be small. This volume is de-pressurized when the on/off valve is closed because any trapped pressure will leak away across the proportional valve. When you next open the on/off valve (with the proportional valve still closed) the only flow into the trapped volume will be that which is needed to re-pressurize it - about 2% if you were going to ~300 bar. So, if your trapped volume were as small as 20 cc and your cylinder had an effective diameter of, say, 80 mm, then you would experience a drop of less than 0.1 mm at the cylinder rod. There would be a distance amplification through the boom but I'm pretty sure you wouldn't feel that when standing in the basket.

To turn your proportional throttle valve into a pressure compensated unit which can operate without a start-up "jump" you could install what is effectively a direct operated reducing valve between the on/off valve and the throttle. This is what happens: when you first open the on/off valve the throttle valve will still be closed but the reducing valve will be spring biased open. Pressure will be applied to the inlet of the reducing valve but its outlet will be dead-headed. As soon as the outlet volume of the reducing valve reaches the appropriate pressure the reducing valve will close. Once again the closed volumes are small (and the pressure rise will also be small) so the compressibility flow will be tiny and the drop in cylinder rod position will be insignificant.

When you eventually start to open the proportional throttle valve the reducing valve will also open and "attempt" to hold the throttle valve inlet pressure at the appropriate value. By this means the pressure drop across the throttle valve remains relatively constant regardless of the magnitude of the load. A constant pressure drop across a [not changing for the moment] throttle valve setting equates to a constant flow rate. The throttle valve inlet pressure would be the setting of the "reducing" valve (which could be fixed, "tunable" or fully adjustable) and the throttle valve outlet pressure would be whatever you have in the return line. A varying back pressure in the return line can be further compensated by connecting the spring chamber of the "reducing" valve to a point immediately downstream of the throttle.....or you could just use a proportional throttle valve that had a built in pressure compensator and save yourself the hassle. One advantage, however, of the home brew approach is that you can set the throttle valve pressure drop particularly low in order to utilize the full opening of proportional valve and to maintain full speed up even when very lightly loaded.

Personally, I'm not convinced about piloting open an over-center valve with a hand pump or 12 V DC motor driven pump. The over-center valve provides over-running control and protection against hose burst because the pilot line pressure is sourced from the opposite side of the cylinder. In a lot of these applications the proportional DCV provides a meter-in and meter-out control and in the event of a burst hose the cylinder rod starts to move faster than intended (because of the loss of the meter-out flow control). The expanding side of the cylinder volume becomes de-pressurized because the incoming flow from the DCV isn't keeping up with the flow needed for that new speed. The loss of pilot pressure causes the over-center valve to close and that limits the extent of the runaway. But, if your pilot signal pressure is isolated from this scheme then it won't decay in the event of a runaway so the thing doesn't provide any hose burst protection. Other drawbacks are that the pilot pressure needed to open the over-center valve will depend on load... as you are pumping away at the pilot line you won't be exactly sure when the movement is going to start. Then to gently stop you have to de-pressurize the pilot line and the point at which it stops will also be a movable feast.

Actually you would be making a "remote control" version of a FLOW control valve but trying to use a PRESSURE control valve as the recipient of the pilot signal. If that's what you want to do then you need to choose an over-center valve with a very pronounced pressure override characteristic.

Incidentally, how do you cope with the non-conductive boom issue? Is radio control considered secure enough if either the transmitter (in the boom) or the receiver (in the base) has been raised to a high AC potential? Would you need fiber-optic control, or opto-isolators on your signals? Are you using non-conductive hydraulic hoses? I know these weren't your original questions, I'm just curious that's all.

DOL

 

[hydtools]

From a ConExpo 2012 interview with various excavator manfacturers:
Going With the Flow
Smart electronics are boosting productivity and fuel economy up to 11% for excavators from JCB, Savanna, GA, according to Ryan Connelly, product marketing specialist. “One of the things we’ve done is allow for simple improvements such as letting gravity assist boom-down functions,” says Connelly. “Basically we’re just optimizing the hydraulic systems so they don’t restrict the flow back to the tank and that allows faster cycle times and more assistance from gravity to drop the boom down.”

Effectively reducing return flow restrictions to speed boom drop. Getting closer to free-fall. Not related to booms on cheery pickers or other man-lifts.

Ted

 

[hydroman247]

@Oldhydroman

Very interesting read. I am going to try and design the schematic for this cylinder mounted valve this week and see what I can come up and simulate it, if possible. It seems like an ideal solution but will need a lot of fine tuning and as you would know, it may not work exactly as it seems. Need to test it.

The base control are not electronic, they are manual spool controls and the control levers in the cage use the same spools but using coils and of course the electronic side of it can be set differently to the manual control in regards to speed and proportionality.

As for the hand pump, we use a direct tank line and the pressure side of it goes into the main base valve block pressure gallery so it acts like the normal pump so there is no run away and as you stop pumping, the feed to the pilot line stops as well as the annular side of the cylinder is drawing in oil and the pressure drops to 0.

Conductive booms are not part of the design at on these machines. They are not to be used within X meters of live power lines. We do have machines that are designed to go near power lines but they are still conductive booms. They have big thick copper conducting cables connecting all joints on the machine so if an operator bypassed all the safety switches and did swing into a power line, the onboard computers should get away with it but we are yet to test that.

@hydtools

As I suspected. I had heard JCB do something like this but couldn't find any info. What they do is probably not safe for man riding machines. Could just be a proportional valve.

 

[hydtools]

JCB also makes mention of using a regenerative connection and gravity assist to lower a material handling boom. Flow from the cap end of the cylinder is connected to the rod end; this would reduce flow from the cap end back to tank.

Ted

 

[hydroman247]

I have included a regen valve in my preliminary design.

I will upload a picture of my basic idea when I can get onto my computer with simulation software on it. Thanks for all your help guys, especially oldhydroman!

Regards,

hydroman247

 

[Oldhydroman]

You're welcome hydroman247.

Thank you for bringing to the forum a thread with so much meat on its bones

DOL

 

[IceStationZebra]

"Do you see anyway around the viscosity change problem? These machines could be working in -20c or 45c and be running all day."

Warehouse forklifts, especially in EU countries, require that the forks lower at no more than X m/sec. If one uses a simple restrictor the lowering speeds will vary considerably from hot and loaded to empty and cold. To get around this issue mose OEMs use a tuned variable restrictor that is velocity sensitive. They can be tuned by adjusting the hole sizes and spring rate.

Some forklifts also have blowout valves on the lift cylinders, but I have no experience with these.

ISZ

 

[hydroman247]

Here is the most basic mock up I did in 10 minutes. Had no time to get it working properly.

Maybe next week I will have some time to sort it out.

 

[hydroman247]

@ISZ

Yes I understand this. Currently without pressure on the annular side of the cylinders, they cannot move. This is part of the safety design for man riding machines. Most likely I will have to use a blowout valve. I am referring to hose burst protection if that is what you mean. Sudden high flow locks the valve shut.

 

[hydroman247]

Some bad news.

On further investigation, direct electronic control is not considered safe for this sort of application. Pilot operated is the only way to go which means this is going to be more complicated and require custom cartridges to be developed and made.

I am still thinking of incorporating electro-proportional control for ease of tuning and setting up but it will have to be in-direct. The regulations for telehandlers and other non-man riding mobile hydraulic machines are very different to my desired application.

There are some pilot operated, pressure compensated available but I will need to do some more research.

Regards,

hydroman247