Hydraulic cylinder end position situation

[JerinG]

I could improve in hydraulic systems, but when I design a machine, I sometimes need some details regarding this area and now is this situation.

Let's say we have a machine consisting of a fixed structure and a moving structure. We move the second structure relative to the first one with a hydraulic cylinder. Now, we have two options for how to fix the second structure in the end position:
1: We don't use a total stroke of the cylinder, but instead, we design both structures so that they have some common points that come in contact before the cylinder reaches the end position. Both structures are somewhat locked to each other. Of course, we need to keep the pressure in the hydraulic system. We also need to design structures strength-wise so that they can withstand these contact forces.
2: We use the full stroke of the cylinder, and there are no common contact points on both structures. We lock the structure in the end position with the cylinder end position. Of course, we keep the pressure in the hydraulic system.

Cylinders are slow-moving; no dynamic or inertia forces are coming into play.

Which one do you prefer and why? Intuitively, I like the first one more.

 

[BEMPE16524]

Can you give a simple sketch of what you plan to do?
how big is your machine? its function?
estimated weight... load to be carried... etc...

R.Efendy

 

[FluidPowerUser]

"Cylinders are slow-moving; no dynamic or inertia forces are coming into play" - Wanna bet? If it's moving, there will be some forces.

You can either take up the forces within the structure or you can absorb the forces in the gland of the cylinder.

If your cylinder manufacturer has done a good job, the gland on the end of the cylinder will be capable of withstanding the full force at the maximum working pressure. The downside of stopping the stroke with the cylinder is that there is no room for flexibility. If more stroke is required, a new cylinder will be required. If you stop the stroke with a mechanical device, it can be moved, but it needs to be strong enough to resist the cylinder forces. That is the total cylinder force, push force + inertia.

 

[JerinG]

Ok, I'm attaching a quick sketch of how I plan to approach this.

In the sketch, you can see the side view of one part of the machine, which has a total length of around 6 m and a height of 1 m. It is a support bench for cylindrical pieces (weight up to 70 kN, length 6 m). We use the bench part of the machine for front-back and up-down manipulation of the part.
In the front and up position, we do some machining on one end of the part (groove grinding and cutting a short piece from the blank) and then move the part up and front to repeat the operation.

I'm still in the design phase, but as I mentioned, I'm choosing between two options. I see the first option as a better one because we still have some manipulation possible in the cylinder for different reasons (tolerances, deformations, etc.), and the structure becomes more rigid during the machining phase than it is if only cylinders are at the maximum extension. I have a position for the first contact point (front-back cylinder) but am still looking for the clever position of the second contact point (up-down cylinder) because there I have a linkage mechanism, which produces big forces in the up direction, so I want to transfer the forces in the cylinder actuating direction.

 

[Doug Hunter]

Either approach can be valid. It will come down to the details. I suggest doing a pro/con/risk analysis/comparison of the 2 methods in your design situation to help you decide.

Best regards,
Doug Hunter
Altarium Technical Consulting

https://www.altarium.us/

 

[JerinG]

Okay, thanks for the input. I'm not very skilled in hydraulic systems details, so I was wondering what the second option could cause to the hydraulic system or if there could be any potential problems. That's why I asked under this part of the board. Any suggestions are still welcome because I'm still designing the bench.

 

[FluidPowerUser]

From a purely hydraulic point of view, you can go with a simple pump, a simple valve and just let the cylinder run to end of stroke or stop it with a mechanical end stop.

Alternatively, you could spend money on a proportional control system that will detect the cylinder position and stop it automatically without applying load to the framework at all.

On this basis, it's about the cost of the mechanical system vs the cost of the control system.

Position control of a hydraulic cylinder is relatively straightforward...if you don't want to be super accurate and / or super quick at stopping in the in desired place repeatedly.

 

[BEMPE16524]

that's basically a very simple system.
As DH said, either one is fine.
Remember to make it as simple as possible.
Yes, you can add so many fancy things, but at the end of the day, the maintenance guy will suffer with all your fancy design.

R.Efendy

 

[JerinG]

Yes, I'm working on as simple as possible. That's usually my goal.

One question, maybe for you hydraulic engineers: What is the most practical and robust way of position control for hydraulic cylinders? We sometimes need it on other machines, and any information here is highly welcome.

 

[BEMPE16524]

Are you talking about the control station of the hydraulic system? When you do high pressure, better to put them away from your main machine. Using a remote control box is another option.

R.Efendy

 

[JerinG]

No, let's say that we need to stop the cylinder at a defined position - at some extended length or if there are more positions along the total length of the cylinder. How do you practically do it? One way is to have sensors in the cylinder, but that is usually for extreme positions. As in pneumatics, but there is only a signal that cylinder is open or closed, not information about exact position.

 

[BEMPE16524]

Linear transducer or reed witch.
Linear transducer: i have seen the hydraulic cylinder with internal or external linear transducer.

Reed switch: with this method, you need to position the sensor at few major point pf your intended position.
R.Efendy

 

[Doug Hunter]

Like R.Efendy said, reed switches are one option. In a basic sense, to control something (in this case, your cylinder position in between the end stops), you need to be able to measure it. There are a number of ways to do this. Tripping a reed switch is a commonly used tried-and-true method. Other possibilities are optical sensors, hall-effect sensors (for ferrous items), and tooth counter sensors in conjunction with a rack. In a simplified case, if you only need to know when you're at your end positions, you can measure your pump discharge pressure (presumably PRV-limited) or pump motor current, which will spike when you hit your end stops. Flowmeters could also be used, but they tend to be bulkier and more expensive.

Best regards,
Doug Hunter
Altarium Technical Consulting

https://www.altarium.us/