CR4® - The Engineer's Place for News and Discussion®


Previous in Forum: 220VAC to Variable DC Voltage (3.3-100DCV) Transformer  
Close
Close
Close
23 comments
Member

Join Date: Dec 2017
Posts: 5

Hydraulic Actuators

12/07/2017 2:45 AM

I am currently doing one project on hydraulic actuator synchronization using mechanical coupling( rigid coupling) as part of my internship. it is cost effective, don't have any complicated controls. just pump the fluid. since it is rigid and the mass is centralized the volume and pressure should be equal to both actuators.

This is the basic structure -->

when I was doing my research, I came across so many models which using other techniques like servo valves, but I hardly saw anything about the mechanical coupling., What are the disadvantages of this system?

PS- the connection is rigid, the weight is centralized, might have slight leakage (assuming, since it is for practical use).

Register to Reply
Pathfinder Tags: actuator fluid hydraulics Mechanics
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
3
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 20033
Good Answers: 753
#1

Re: hydraulic actuators

12/07/2017 4:59 AM

One disadvantage might be if the two items to be synchronized are too far apart to be easily linked mechanically.

Equal-flow dividers and other devices can help to correct this, but small leakages or obstructions can lead to slow drift. Various types of end-of-stroke relief valve schemes can fix that, too, but only if you attain end-of-stroke from time to time.

Your question is a good one.

__________________
In vino veritas; in cervisia carmen; in aqua E. coli.
Register to Reply Good Answer (Score 3)
Guru

Join Date: Apr 2008
Location: Chicago
Posts: 2297
Good Answers: 73
#2
In reply to #1

Re: hydraulic actuators

12/07/2017 6:51 AM

GQ

GA

__________________
High Tolerance is Beautiful
Register to Reply
Guru
Canada - Member - If there is a way to screw someting up, there is someone to do so! Safety - Hazmat - New Member

Join Date: Mar 2007
Location: Iqaluit, NU. Canada
Posts: 1815
Good Answers: 137
#3

Re: Hydraulic Actuators

12/07/2017 9:12 AM

IMHO... no matter what you do there is going to be slight differences in linear motion (speed) of the cylinders as they react to the pressure of the fluid pushing on them.

What this means is that the connection between the cylinder rods and the platform lifting the load will have to have a pivot point of some kind located where the rods connect to the platform otherwise it will more than likely end-up bending and or snapping the rods.

This will cause the platform to tip a little bit one way or the other, but, I think that flexibility will be needed.

__________________
Joe Contractor to Electrical Inspector, "What do you mean you are going to make me follow the code?".
Register to Reply
Guru

Join Date: Oct 2008
Posts: 40050
Good Answers: 1598
#4

Re: Hydraulic Actuators

12/07/2017 9:30 AM

Unless you have exactly the same pressure, acting exactly in unison you will get binding of one cylinder as unequal forces are applied to the platform.

The platform will need guide rods such as these.

Round Tie-Rod Series - LMG Presses

Or a flexible attachment point or points.

Register to Reply Score 1 for Good Answer
4
Guru
Engineering Fields - Control Engineering - Time to take control United States - Member - New Member Engineering Fields - Systems Engineering - New Member Engineering Fields - Mechanical Engineering - New Member

Join Date: Apr 2009
Location: Tampa, Florida, USA
Posts: 2053
Good Answers: 84
#5

Re: Hydraulic Actuators

12/07/2017 9:36 AM

Most of the following I pulled from Fluid Power Systems and Circuits by Russell W. Henke

The circuit shown seems as if it would provide what’s desired (synchronized movement), however, even if the two actuators are made the same (same part number, same manufacturing process….they will still likely have different friction, machining dimensions and leakage) the loads on the two piston rods would also have to be identical for perfect synchronization. The slightest imbalance and the cylinder with the lower load reaction would extend first because it would require the lower operating pressure to move it. Also because of variations in manufacturing, the two cylinders have different responses, so even if they started out in sync the y would get out of phase.

One open loop method is to use a circuit like this:

Cylinders 1 and 2 are connected in series. The fluid flows from the pump through the directional control valve to the cap end of cylinder 1. The fluid in the head end of cylinder 1 flows into the cap end of cylinder 2. For approximate synchronization, the head end displacement of cylinder 1 must equal the cap end displacement of cylinder 2. The pump must be capable of developing pressure equal to the sum of the pressure differentials across both pistons.

This circuit will not guarantee synchronization over a large number of cycles because of the differentials in leakage of the two cylinders (as Tornado mentioned). Methods to re-sync can be incorporated such as mechanical stops and way to replenish system fluid that has leaked while both pistons are against their stops.

The only method to ensure absolute synchronization is the use of a servo system.

__________________
J B
Register to Reply Good Answer (Score 4)
Guru
Canada - Member - If there is a way to screw someting up, there is someone to do so! Safety - Hazmat - New Member

Join Date: Mar 2007
Location: Iqaluit, NU. Canada
Posts: 1815
Good Answers: 137
#6
In reply to #5

Re: Hydraulic Actuators

12/07/2017 1:18 PM

I can see bleeding and filling the closed section would be a nightmare.

__________________
Joe Contractor to Electrical Inspector, "What do you mean you are going to make me follow the code?".
Register to Reply
Guru
Hobbies - CNC - New Member Hobbies - DIY Welding - New Member Engineering Fields - Electromechanical Engineering - New Member

Join Date: Aug 2007
Posts: 19134
Good Answers: 341
#7

Re: Hydraulic Actuators

12/07/2017 2:38 PM

About 25 years ago.... I had seen a hydraulic schematic and a missile launch platform for dual cylinder synchronization. It was in a suppliers catalog. It was surprisingly simple. I had Xerox that for future use... I'll see if I can reclaim it in my files.

__________________
phoenix911
Register to Reply Score 1 for Good Answer
Member

Join Date: Dec 2017
Posts: 5
#9
In reply to #7

Re: Hydraulic Actuators

12/08/2017 4:35 AM

did you got it yet?.....

tq

Register to Reply
Guru
Hobbies - CNC - New Member Hobbies - DIY Welding - New Member Engineering Fields - Electromechanical Engineering - New Member

Join Date: Aug 2007
Posts: 19134
Good Answers: 341
#10
In reply to #9

Re: Hydraulic Actuators

12/08/2017 7:43 AM

Not yet,.. This weekend I'll look... you have to realize, there's 2o years of info. Hopefully its in the basement storage, and not in any of the 3 storage lockers I have.

__________________
phoenix911
Register to Reply
Member

Join Date: Dec 2017
Posts: 5
#19
In reply to #10

Re: Hydraulic Actuators

12/12/2017 8:47 PM

sir, any updates?

Register to Reply
Guru
Hobbies - CNC - New Member Hobbies - DIY Welding - New Member Engineering Fields - Electromechanical Engineering - New Member

Join Date: Aug 2007
Posts: 19134
Good Answers: 341
#20
In reply to #19

Re: Hydraulic Actuators

12/12/2017 8:51 PM

when through my basement and one storage locker... (needed straightening up) I have 2 storage lockers to go, and I don’t thing I’ll get to it anytime soon.

__________________
phoenix911
Register to Reply
Member

Join Date: Dec 2017
Posts: 5
#21
In reply to #20

Re: Hydraulic Actuators

12/12/2017 8:55 PM

chey.... nvm,

thank you for your effort,!!!

Register to Reply
Guru

Join Date: Dec 2010
Location: North West England
Posts: 928
Good Answers: 117
#8

Re: Hydraulic Actuators

12/08/2017 4:17 AM

If the load is always centred and the load platform is rigid enough to prevent distortion between the two cylinders, why use two cylinders? Place a single larger cylinder under the centre of the platform. For greater stability take a look at how scissor lifts operate.

Register to Reply
Active Contributor

Join Date: Jul 2017
Posts: 10
#11

Re: Hydraulic Actuators

12/08/2017 8:06 AM

A flow divider/combiner is also a good idea in this type of circuit. It can be integrated as your basic control valve as well. In the picture below the pressure hose coming from your pump would hook up to the bottom and each cylinder would get a pressure hose from the top then the return from each cylinder would run back to the tank.

This device does a good job at equally distributing the flow to your cylinders/motors for synchronized movement speed.

http://www.hydraulicspneumatics.com/other-technologies/book-2-chapter-11-flow-divider-circuits

Register to Reply
Guru
Hobbies - CNC - New Member Hobbies - DIY Welding - New Member Engineering Fields - Electromechanical Engineering - New Member

Join Date: Aug 2007
Posts: 19134
Good Answers: 341
#12
In reply to #11

Re: Hydraulic Actuators

12/08/2017 8:17 AM

that looks familiar...

__________________
phoenix911
Register to Reply
Active Contributor

Join Date: Jul 2017
Posts: 10
#13

Re: Hydraulic Actuators

12/08/2017 8:18 AM

This is also a good mechanical means of keep the load balanced as the cylinder will naturally move in sync as limited by the mechanism.

Register to Reply
Guru
Hobbies - CNC - New Member Hobbies - DIY Welding - New Member Engineering Fields - Electromechanical Engineering - New Member

Join Date: Aug 2007
Posts: 19134
Good Answers: 341
#14
In reply to #13

Re: Hydraulic Actuators

12/08/2017 8:22 AM

cylinder will naturally move in sync as limited by the mechanism.

That is where problem lays, where your valve limits the binding by the mechanism.

__________________
phoenix911
Register to Reply
Active Contributor

Join Date: Jul 2017
Posts: 10
#15
In reply to #14

Re: Hydraulic Actuators

12/08/2017 9:33 AM

This design at least puts the mechanical coupling very close together and not on each end of lift platform making following much better.

Register to Reply
Guru
Hobbies - CNC - New Member Hobbies - DIY Welding - New Member Engineering Fields - Electromechanical Engineering - New Member

Join Date: Aug 2007
Posts: 19134
Good Answers: 341
#16
In reply to #15

Re: Hydraulic Actuators

12/08/2017 10:09 AM

That is good to point out...

__________________
phoenix911
Register to Reply
Guru

Join Date: May 2016
Posts: 916
Good Answers: 44
#17

Re: Hydraulic Actuators

12/08/2017 12:00 PM

The big issues with the design you present are lateral forces and binding. With an eccentric load as shown, during a lift, the left cylinder will lag behind the right cylinder. The arrangement shown causes both cylinders to exert the same force, but the left cylinder carries greater load. You can expect the table and attached pistons and rods to want to rotate counterclockwise and apply lateral loading on the left side of the piston rod packing glands and seals and on the right side of the pistons.

Elsewhere in the thread there is a circuit where the cylinders are connected in series. This is a better arrangement unless one of the cylinders develops a leak in the section of the circuit between cylinders. Then the cylinders lose synchronization and you get back into lateral loading and binding on the cylinders. A remedy would be to add bypass relief valves on both cylinders set about 10% above maximum load lift pressure so that you can periodically drive both cylinders to end of travel, lift both relief valves and re-zero the synchronization.

Another solution would be to add external mechanical guidance to the table and make the cylinder mechanical connections flexible.

Register to Reply
Member

Join Date: Dec 2017
Posts: 5
#18
In reply to #17

Re: Hydraulic Actuators

12/12/2017 8:45 PM

actually, the mass is always centered through another mechanical set-up.

and about the series connection even though both the actuators in sync, but the pressure difference and the force generated will be different right? because of the diameter difference....!!

I know if I'm using a flow divider or a servo valve it will be in sync. but I'm doing a research( i.e. I'm not trying to find new means to synchronize them, I just wanna know why this rigid coupling can't do it) in theory, this system is capable of synchronizing itself without any external control. but because of some reason, this is not using in heavy load lifting. instead, the industry using more complicated and expensive servo valve systems........

I want to know the reason, that's all

thanks for your comment....!!!!!!!

Register to Reply
Guru

Join Date: May 2016
Posts: 916
Good Answers: 44
#22
In reply to #18

Re: Hydraulic Actuators

12/13/2017 11:42 AM

Good point that you make about areas. The area of the working fluid on the connected cylinder volumes needs to be the same. A method to do this with identical cylinders is to mount one with the rod up and one with the rod down.

With two cylinders connected in parallel as in the first circuit, there are several other sources of lack of sync. In a hydraulic system, whichever cylinder works against the least force moves first. Your number one source of mismatch will be in the seals. One will come loose first and move first until it picks up enough load that the second seal breaks loose. A moving seal generally has much less drag than a stationary seal. The source for the additional needed force will be racking of the mechanical rod connections and lateral force on the pistons and piston rods. The racking is also going to contribute to binding in the pistons and rods. All in all, the mechanical connections will have to be way overbuilt as will the piston rods to keep the system from binding and failing. All of this happens even if the working load is perfectly balanced at the start of the cycle.

I would suggest you do a free body analysis of the cylinder and table assembly and an evaluation of clearances and forces at the pistons, piston rod glands and table joints to get a good picture of how one stuck piston seal will cascade a failure through the mechanical structure. You can also determine how the seal clearances are affected by racking, leading to premature hydraulic seal failures.

Register to Reply
Guru

Join Date: Dec 2010
Location: North West England
Posts: 928
Good Answers: 117
#23
In reply to #18

Re: Hydraulic Actuators

12/13/2017 12:15 PM

The reason why more complicated control systems are employed is that the system as originally described does not work. A flow divider is just a pair of metering pumps attached to a common shaft and they do not solve the problem in a system with rigid linkages because they do not address the basic problem which is "positive feedback". A flow divider will work in an installation where the linkages are not so rigid as it will limit the out of balance to a lower value than the slack in the linkage can tolerate. Where a flow divider is used it is important to bottom the cylinders against an end stop at the end of each cycle to prevent imbalances in each cycle being superimposed and creating what is known as "progressive tolerance build up". No matter how carefully you set the system up there will always be a small difference in friction between the two sides. It may be in the cylinder seals, machining tolerances, mechanical linkages or elsewhere but it will always be there somewhere. This will cause one cylinder to lag by a fraction and that forces more oil into the lead cylinder, which magnifies the problem and makes the imbalance worse. The larger imbalance causes more magnification until a snowball effect occurs causing the cylinders to lock up. Servo valves or complex hydraulic solutions introduce "negative feedback" where sensors register if one cylinder starts to lead, then the flow to that cylinder is cut back until a re-balance is attained. Servo systems are difficult to set up. If too much correction is applied then the system will overshoot, the correction is then reversed and it overshoots in the other direction. This is called "hunting". Read up on PID loops to see how this can be corrected.

Register to Reply
Register to Reply 23 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

JBTardis (1); JE in Chicago (1); jhhassociates (2); Jpfalt (2); khelectrical (3); livingstonax (4); lyn (1); North of 60 (2); phoenix911 (6); Tornado (1)

Previous in Forum: 220VAC to Variable DC Voltage (3.3-100DCV) Transformer  

Advertisement