Twin hydraulic cylinder on agricultural equipment. Will they work as you hoped?
There are a few ways of controlling multiple cylinders in different applications. If you simply connect them up, there is every chance they won’t work or not as expected. Before jumping directly to flow dividers and position sensors, is there a simpler way? What are you trying to achieve? There are people who have simple requirements and go with easy and inexpensive solutions before running with complex technology.
If you are controlling one single cylinder, life is relatively easy. If it is a single acting cylinder, pressurise it and it will carry out its work cycle as long as the pressure is high enough. Moving a load, aligning components, a metal forming operation or a hundred other applications. You need spring or gravity return, as discussed in our very good white paper on why we always look at single acting cylinders first before moving on to anything more complicated.
The same is true in using double acting cylinders. A single double acting cylinder needs more valves to provide control compared with single acting. But they work well in the right application. Read more on basics.
What is difficult about controlling multiple cylinders?
The bottom line is making sure that they work as you intended. You will have gone to multiple cylinders for a reason. Are you running multiple cylinders at the same time or a sequenced operation where one runs after the other. Maybe you are doing multiple tasks from the same pump. For example, controlling three separate single acting cylinders all doing different jobs. Or possibly just one task but your design simply doesn’t allow the load to be exerted at one point. You need two, three or four cylinders. A good first question is whether the cylinders are stand alone or are they synchronised in some way? Do they interact with each other? The moment you determine that they interact with each other, you will think about ensuring they run in tandem mechanically or hydraulically.
Let’s look at a Tail Lift for vehicles as an example. This is a massive hydraulic market. A typical unit might have two cylinders for lifting and two for tilting. What is the relationship between the cylinders? Typically, look at the Palfinger MBB PTC100 unit tail lift, just as an example. Your two lifting cylinders will be running synchronised in tandem. The tilt cylinders will be running at a different time but again in tandem. We aren’t experts, but the web suggests that the two lift cylinders are kept running in tandem by a flow divider valve. We have seen the parallelogram arms used under tail lifts. Is there any bracing between the arms to mechanically keep them synchronised if there is on cylinder per arm, or does the design rely on hydraulically splitting the flow? The platform is wide and with very little depth is available, which is probably a basic challenge to be addressed by designers. Tell us.
This Palfinger tail lift uses two lift cylinders and two tilt units.
An interesting design. Image by courtesy of Palfinger.
The worst that happens is that multiple single or double acting cylinders don’t move at the same rate, as the loads vary. That could make an unstable loading dangerous or jam a mechanism and cause damage. The same is true on releasing the pressure from multiple single acting cylinders. You simply find that the flow rates are different and something retracts or lowers unevenly, tipping a load or jamming a mechanism. The higher the pressure, the more it jams.
Rather than diving in from the most complicated and expensive side, we will tackle this from cheap and easy. Let’s look at some low -tech applications.
How to control up to four single acting cylinders independently using one pump and single lever control.
Larger yachts commonly have this challenge. A backstay tensioner, boom outhaul and kicker all needing their own cylinders. These are all independent systems where a single acting cylinder can be loaded up then left pressurised while the crew moves to another circuit that needs tweaking. Look at catalogue panels from Harken or OMS. We manufacture our panel pump Micropac type MPP for integration into these units. Our miniature single lever hand pump also suits this specialist application for very small micro high pressure hydraulic systems on applications like high tech sailing dinghies or keelboats.
How do you use these simple systems? The hand pump (or powered pump) has a hold/release valve and pressure gauge on the outlet and the feed is to a selector valve. One flow in and up to four out. For catalogue valves, look at the Hoke Multimite offering to 400 bar or the sub miniature Swagelok 43Z selector for up to 172 bar and low flow rates. The main requirement is that whilst switching flow paths, there is no intermingling, or you would struggle with pressure drops. On the little Swagelok unit, you need to order a special version to address this instantaneous mixing of flows. These specialist selector valves allow pressure in and flow out through ports 1, 2, 3 or 4. You pressurise or vent any of the four cylinders using the hand pump (or powered pump), hold/release valve and the pressure gauge. Once the cylinder is set, move to another flow path and adjust that.
This is a great solution for controlling multiple single acting cylinders where all four are 100% independent.
Turn selector valve to select which of your four cylinders to actuate. Operate pump and adjust hold/release to set position. Then select another independent cylinder. Up to four cylinders and four different jobs.
What is the easiest way to run single or double acting cylinders synchronised?
Lots of applications simply don’t have room for a single cylinder centrally exerting the load. The layout might lend itself to having two cylinders outboard and doing the work. The simplest fix is to mechanically brace them and keep them in tandem and perfectly synchronised. That is fine provided the bracing is strong enough. As we have said often, be aware of the enormous loads that hydraulics can generate. The nightmare scenario is a twin cylinder mechanism using bracing that jams because of flexing then locks up harder and harder as pressure is increased, causing damage or danger to life. People who use bracing cheaply and most successfully are probably industries that have been doing it for years and have a well proven design family.
Does a pressure compensated flow control help on synchronised cylinders?
Yes, we would say they can help. If you have two synchronised single acting cylinders with gravity or spring return, you might use two pressure compensated flow controls on the flow feed back to a hold release valve to just keep the retraction rate balanced. We have only used them on retract. Nasty jams on mechanisms can happen on advance or retract, so we are only covering one cycle here.
Is there an easy way to synchronise cylinders at low flow rates? What about water hydraulics and 316 stainless equipment?
There are solutions for synchronising cylinders at higher flow rates on oil. Very early in our business 40 years ago, we had a nightmare with low flow rates applied to proprietary splitters. Our solution then and now for two synchronised cylinders is a tandem hand pump. Two separate double acting (i.e. pumping on up and down stroke) hand pumps integrated into a single compact body with a single common clevis. The MH series Micropac pump is a neat and cost effective solution. Then connect in ball release valve or four way directional valves with the levers ganged mechanically. We have always manufactured our own linkage to gang the valves. Plus, pressure compensated flow controls on the return line if you are on single acting cylinders. This is a great low tech solution even for low flow rates, fluids like water or 316 stainless kit. Catalogue flow splitting solutions probably won’t work for you in these low flow applications. Customers have sometimes bought two pumps, mounted them side by side and bought a single customer handle to pump them in tandem. That’s another solution. We can offer this. A little less elegant and rather agricultural, but the engineering works.
Our Micropac MH Tandem pump. Two independent pumps in one body.
A unique solution for synchronised flows and also invaluable for use with pressurised inlets, for example refilling fire extinguishers with pressurised carbon dioxide CO2.
The MH series Tandem pump integrates two double acting (pumps on up and down strokes) into one body. Connect Cylinders to each port. Use ganged hold/release valves. Plus, a pressure compensated flow control valve on each feed to release to keep the returns in sync.
Use the same MH series tandem hand pump but connect to ganged four way valves to keep two double acting hydraulic cylinders running in sync.
How do I synchronise cylinders at 200 bar on mineral oil?
If you need to go beyond a MH series tandem hand pump and move to a powered system, then you have new options. Use mineral oil with solenoid valves and you more proven solutions. There is quite a lot of web content. We will briefly describe a few but talk to the specialists.
Will a flow divider from a single pump work for me to synchronise cylinders?
Cartridge valve suppliers do list flow splitters as a product. They obviously work. People talk of them not being very precise on the very low flows from a hand pump. Ask your cartridge valve supplier. Another solution is a gear pump rotor-based splitter where two flows are metered using two gear rotors side by side. We have no experience of these. All gear rotors have some level of leakage inherent in the design. Again, ask your supplier.
A Wavoil gear flow divider. Copyright Wavoil.
Will a split flow pump allow me to synchronise cylinders?
You can buy split flow pumps where a single motor drives two pumps. Two individual gear pumps driven from one shaft are one option. That is probably a cost effective option. Something more precise but expensive is moving to multiple piston pumps. We are big fans of the US Dynex Rivett pumps and note that they can split the pistons between alternate circuits. Talk to them.
Dynex Rivett offer excellent split flow piston pumps. Watch their video.
What other parts do I need in a hydraulic split flow circuit?
If you are using a split flow pump, using it with double acting cylinders and solenoid valves should on the face of it be sound. You still have to confront the implications of loads being significantly different on two cylinders. Again, specialist advice might be in order from somebody impartial.
Why would I integrate position sensors into multiple cylinders that I wish to synchronise?
A considerably more complex circuit uses position sensors for each cylinder then servo valves to control the flow to each. That is a continuous sensor to monitor position rather than the very simple end position indicators that are required for lots of applications. We have seen an Enerpac “Synclift” system for load lifting using multiple synchronised cylinders in their catalogue. That is pretty sophisticated.
There will be people who use cylinders with position sensors in mainstream industrial applications such as automation. That is quite specialist, although it will be quite a large market. Again, talk to an expert.
The Enerpac Synclift system has specialist lifting applications. Watch their video.
In conclusion, running multiple cylinders is a defined need in hydraulics. You need to be very sure what you actually need. There are some clever and cost effective solutions for multiple cylinders, even on very low flows, water/low viscosity fluids and 316 hydraulics. Move to synchronising cylinders and there are some basic easy solutions before you dive into catalogue offerings. We have offered some proven solutions. Often, the winning design offers a clever solution.
Talk to Sarum Hydraulics. After over 40 years, we are the hand pump experts.
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