If you, like a lot of guys out there, are wondering what the differences between double-acting & single acting hydraulic cylinders is, it’s really pretty simple. Double-Acting means “power up/power-down” (or in/out) while Single-Acting means “power up/gravity down”. That’s it (in theory). It seems pretty simple, but there are a few things you need to know before you decide what is best for your application. Here are my 5 top reasons you go with double acting over single-acting hydraulic cylinders.
1 – Much better availability
Almost all standard cylinders are double acting. While it is it is pretty easy to convert a double acting cylinder to single acting, it is still a double acting cylinder and designed for the most part to be used as such. Many of the inexpensive import cylinders have an o-ring piston seal, which is great when there is pressure on both sides, but can roll out and cause the cylinder to stick if it’s used as a single acting. Custom made single-acting cylinders have a smaller piston, and a single lip seal, made just for pushing. If a double-acting cylinder has a crown or Hallite 775 seal, it will work well in a single acting application, but sometimes the fit is so tight that is might not be terribly smooth on the way down.
2 – It always comes down.
Since the cylinder is being powered up and down under pressure, you don’t have to worry about not having enough force to bring it back down. I’ve seen this hundreds of times on dump trailers, in particular. Most of the time, the single-acting cylinders work just fine, BUT if it ever sticks, it is a pain in the rear to bring it back down, usually requiring a hand winch or one of your buddies hang on the sucker while you are pushing the down button.
3 – Smaller hydraulic fluid reservoir.
Double-Acting cylinders act as a reservoir, because fluid flows under pressure to the piston side to extend the hydraulic cylinder. On the return stroke, the fluid flows under pressure to the rod side of the cylinder in order to retract it. This means that the cylinder is holding most of the fluid, so if you have a limited amount of space in your application, such as is usual in applications with DC hydraulic power units, you can use a much smaller tank to hold the oil.
4 – Most of the time, it’s the only viable option.
In a lot of situations, a double acting cylinder is the only thing that makes sense. These include logsplitters, steering units, digging tools, anything that turns or anytime that the power of gravity isn’t avialable. The only time a single acting cylinder is usuful it when you want to lift something, and you know the weight on the cylinder will be enough to bring it back down. You might choose single acting then, usually because the cost of the circuit is a little cheaper, since a single acting power unit has less valving and you have to pay for 1 less hose.
5 – Corrosion
Think about it. On a single acting cylinder, the rod is never lubricated & since there has to be a breather on some sort on the rod end, you have the possibility to introduce moisture into the cylinder. Not only is the rod more susceptible to corrosion, but so is the inside of the tubing, which is not chrome plated. This might not be an issue if it’s used regulary, but what if it’s being stored outside during the winter? Not only is corrosion bad for the intergrity of the steel, It will destroy a piston & wiper seal pretty quickly, which means you’ll have leaks & less power for lifting.
Don’t get me wrong, there’s a place for both types of cylinders and in the right situation it makes sense to use gravity power. If you have or are building an auto-lift, pallet lift, elevator, man lift or anything of that sort taht will go straight up & down, be reasonalby well maintained & protected from the elements. Single acting hydraulic cylinders also use less battery power in a DC application which might makes sense in some situations, but I always hated gravity-down dumps, as they just have too much potential for failure, in my opinion. So, there you have it. Agree or disagree, that’s my 2 cents.