Question on Forklift Hydraulics

Does anyone have a table listing typical number/size of hydraulic cylinders by forklift capacity?

I'm primarily interested in the diameter of lift cylinders; I am trying to get an understanding of how that changes as lift capacity increases. A table would be perfect, either general in nature or manufacturer specific, but if you just have an example or two, that would help as well.

Thanks!

Is there a purpose to the question? Lift cylinders vary not only according to weight capacity but also according to lift height. A multi-section mast requires a very different cylinder than a low-lift unit.

With hydraulics, given the same pump pressure a piston with a larger surface area has a greater capacity. Plenty of basic hydraulic tutorials online.

I'm working on an idea for a related product, and I'm just trying to get a general understanding of what is used at different lift capacities. I know it varies by type or class of truck as well, but in general, I know that for two and a half ton through four tons, it is generally a 3" or 3 1/2" cylinder with two smaller secondary cylinders, right? And that six to eight tons would have a 6" cylinder? I think those numbers are for the engine powered forklifts (class 4/5), right?

First, you have to figure the area of the cylinder. 3.14 X radius squared
Then multiply that number by the psi of the hyd system. That will give you the lift force.
You also have to keep in mind that most forklifts use lift chains to increase lift of the carriage. If the forklift lifts with lift chains, your cylinder force will be cut in half but the carriage will move twice as far as the cylinder and twice as fast.

Example- 3" cylinder, 2,500 psi hyd pressure
R= 1.5"
R squared= 2.25
3.14 X 2.25= 7.065 cylinder area
7.065 X 2,500 pis hyd pressure= 17,662.5 lb
Forklift with lift chains- 17,662.5 / 2 = 8,831.25 lift force on the forks.

Also keep in mind, the cylinder diameter is the actual piston diameter, not the outside. Most often this is also the size of the cylinder bore but every once in a while you'll run across a single acting cylinder that has no piston that seals against the walls of the bore. On these type the force is just applied to the end of the rod. Therefore, the rod diameter will equal the size of the piston on these type.

If you increase the pressure by 50% the force is increased by 50%. Of course every pressure part has a working psi Maximum. The higher the pressure the more expensive and bigger the item.
Bill D.

Bill D said:

If you increase the pressure by 50% the force is increased by 50%. Of course every pressure part has a working psi Maximum. The higher the pressure the more expensive and bigger the item.
Bill D.

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Just to nit pick, wouldn't higher pressure allow the use of smaller items? To lift a ton with 100psi would
require a cylinder over 5" in diameter, while at 1000psi, you would need just a 1.6" cylinder. And at 4500psi,
approximately a 3/4" cylinder...

Now to create and contain and control that pressure, might need some thicker walls and bigger pumps.

m16ty said:

First, you have to figure the area of the cylinder. 3.14 X radius squared
Then multiply that number by the psi of the hyd system. That will give you the lift force.
You also have to keep in mind that most forklifts use lift chains to increase lift of the carriage. If the forklift lifts with lift chains, your cylinder force will be cut in half but the carriage will move twice as far as the cylinder and twice as fast.

Example- 3" cylinder, 2,500 psi hyd pressure
R= 1.5"
R squared= 2.25
3.14 X 2.25= 7.065 cylinder area
7.065 X 2,500 pis hyd pressure= 17,662.5 lb
Forklift with lift chains- 17,662.5 / 2 = 8,831.25 lift force on the forks.

Also keep in mind, the cylinder diameter is the actual piston diameter, not the outside. Most often this is also the size of the cylinder bore but every once in a while you'll run across a single acting cylinder that has no piston that seals against the walls of the bore. On these type the force is just applied to the end of the rod. Therefore, the rod diameter will equal the size of the piston on these type.

Click to expand...

I understand that there is a tradeoff between diameter and pressure. How do most manufacturers resolve that tradeoff? Do they boost increase system pressure? I would imagine that's what they do in a family of products with multiple models with different lift capacities, so as to accomodate the hardware in the same size truck. But what diameter lift cylinders are most generally used for a 2 ton? a 5 ton? a 10 ton? Are they always paired?

And I know that the tilt cylinders would obviously be smaller, but are they generally 2"? Or do they need to be larger on the larger trucks?

I'm trying to understand this from a market perspective, not an engineering perspective. I'm working on an invention that I think would be useful in the industry, but I'm trying to get an idea of what the industry needs.

Most farm equipment is regulated at 2500 to 3000 lbs pressure, some backhoe pressures hit 5000. I believe this is about the top end for most consumer used systems.

ronf said:

Most farm equipment is regulated at 2500 to 3000 lbs pressure, some backhoe pressures hit 5000. I believe this is about the top end for most consumer used systems.

Click to expand...

2,500 psi is what you will find to be the most common. Most off the shelf hyd components are rated for 2,500 with 3,000 usually being a option. When you start getting above 3,000, you are getting into specialized components that aren't as widespread.

We've got a jacking system that runs 10,000 psi. Most of the components look on the outside like standard hyd stuff, but the cost is at least 3 times as much.

Most forklift's limiting factor is the counterweight. Most, in good working order, don't have any problem at all lifting the rear wheels off the ground.

When you get into tilt cylinders, you are talking about a whole different ballgame. How high up on the mast they are mounted, the lower mast pivot point, how far out the load is on the forks, how high the load is lifted, and degree of tilt all contribute to what the tilt cylinders can handle. There is way more that goes into that than just cylinder diameter and system pressure.

Pretty much a forklift as a whole boils down to a simple lever. How they accomplish the simple lever principle into a forklift package can get pretty complicated.

Loud said:

I understand that there is a tradeoff between diameter and pressure. How do most manufacturers resolve that tradeoff? Do they boost increase system pressure? I would imagine that's what they do in a family of products with multiple models with different lift capacities, so as to accomodate the hardware in the same size truck. But what diameter lift cylinders are most generally used for a 2 ton? a 5 ton? a 10 ton? Are they always paired?

And I know that the tilt cylinders would obviously be smaller, but are they generally 2"? Or do they need to be larger on the larger trucks?

I'm trying to understand this from a market perspective, not an engineering perspective. I'm working on an invention that I think would be useful in the industry, but I'm trying to get an idea of what the industry needs.

Click to expand...

Before spending too much time on your invention you need to do thorough research on what already exists. Technically speaking the things we call forklifts are officially known as Lift Trucks because they can mount more than just forks. I have seen them set up with carpet poles, drum handlers, tree handlers for nurseries, and even giant hydraulically operated blacksmith tongs. The operators who set up for trade shows often use special scale trucks that weigh each load and deliver a printout of total weight moved, which is how the customer gets billed.

In general system pressure remains consistent within a model line as it is far easier to use the same pumps and hoses and either upsize the cylinders or double up on them. As mentioned, most lift trucks are physically capable of lifting more than their rated capacity but stability suffers and it is extremely unpleasant when the steering "goes light" due to inadequate counterweight.