Shop Now
-
Hydraulics
- Parts
- Fittings
- Motors
- Filters
- Hose Tails
-
Adaptors
- Tube Weld
- Caps
-
Straight
-
Nipple
- BSPP Male x BSPP Male
- BSPP Male x JIC Male
- BSPT Male x BSPT Male
- BSPT Male x JIC Male
- BSPT Male x UNO Male
- JIC Male x JIC Male
- NPT Male x NPT Male
- ORFS Male x Metric Stud Male
- ORFS Female x ORFS Male
- NPT Male x NPT Female
- BSPT Male x MS Male
- BSPP Male x UNO Male
- BSPT Male x BSPP Male
- BSPT Male x NPT Male
- NPT Male x JIC Male
- JIC Male x ORFS Female
- JIC Male x UNO Male
- JIC Male x ORFS Male
- JIC Male x Metric Stud Male
- UNO Male x ORFS Male
- ORFS Male x ORFS Male
- JIC Male x JIC Female
- BSPP Female x JIC Male
- BSPT Male x ORFS Male
- BSPT Male x ORFS Female
- Metric Stud Male x Metric L Ma
- ORFS Male x ORFS Fixed Female
- JIC Male x Code 62
- Metric L Male x Metric L Male
- Reducing Bush
- Socket
- Bulkhead
- BSPT Male Suction Tail
- UNO Male Suction Tail
-
Male x Female Swivel
- BSPP Male x BSPP Female
- BSPP Male x JIC Female
- BSPP Male x ORFS Male
- BSPT Male x BSPP Female
- JIC Female x ORFS Male
- BSPP Female x UNO Male
- JIC Female x UNO Male
- BSPT Male x JIC Female
- ORFS Fixed Female x ORFS Male
- JIC Male x JIC Female Restr
- BSPP Male x ORFS Female
- JIC Male x BSPP Female
- JIC Male x JIC Female
- BSPP Male x Metric H Female
- BSPT Female x BSPP Male
- BSPP Male x Metric L Male
- BSPP Male x Metric L Female
- BSPP Male x Metric H Male
- BSPT Female x BSPT Male
- OEM Female x BSPP Male
- Male x Female Solid
-
Nipple
- Plug
- Tee Piece
- Cross
- Flange Clamps
- Banjo Bolts
- Tube Fittings
- 45 Degree Elbow
-
90 Degree Elbow
-
Elbow Block
- BSPP Male x JIC Male
- BSPT Male x JIC Male
- BSPP Male x ORFS Male
- BSPT Male x BSPP Female
- Metric L Male x Metric L Femal
- NPT Male x NPT Female
- JIC Male x Metric Male
- BSPP Female x UNO Male
- JIC Male x UNO Male
- BSPT Female x BSPT Female
- BSPP Male x BSPP Female
- BSPP Male x BSPP Male
- BSPT Male x BSPT Female
- BSPT Male x BSPP Male
- BSPT Male x UNO Male
- BSPT Male x BSPT Male
- BSPT Male x Suction tail
- NPT Male x JIC Male
- JIC Male x JIC Female
- JIC Male x ORFS Female
- JIC Male x JIC Male
- UNO Male x ORFS Male
- UNO Male x Suction Tail
- ORFS Male x ORFS Male
- NPT Male x ORFS Female
- ORFS Female x ORFS Male
- Elbow Tube
- Bulkhead
-
Elbow Block
- Hose
- Valves
- Pumps
- Cylinders
- Power packs
- Breakaway Couplings
- Pipe Clamps
- Seals
- Ferrules
- Equipment
- Hydraulic Hose Kits
- Misc
Blog Archives
Blog Categories
Upcoming Events
Just Arrived
What's New
Follow Us
Latest Posts
Over-Centre Valves - how do they work?
Over centre, valves can be described as a type of pilot assisted relief valve, with the only difference between the two being the check valve will open fully when sufficient pressure is applied with pressure in the cylinder port being the only restrictive force, whereas the over-centre valve has to overcome force from the spring mechanism which is reduced by load pressure.
There are 3 main areas of load based functions the over-centre valve provides, which are applicable to both rotary and linear load motion. These areas comprise:
Controlling load – This involves the valve ensuring that the actuator doesn’t run ahead of the pump, thus reducing the risk of cavitation by controlling load-induced energy and preventing loss of control.
Ensuring load safety – This safety measure controls movement and ensures that load is under control when a component malfunction occurs, such as a hose failure.
It also ensures that people, equipment and property remain safe when heavy machinery is used, such as a crane with a boom, which has the potential to cause substantial damage if control is lost.
Holding load – Working with the directional valve when it is situated in the neutral position, the load holding function of the over-centre valve prevents any movement of the load and also prevents leakage past the directional valve while it is in the closed position.
Pilot ratios explained
When a system is in the design stage, pilot ratios are the main factor that needs to be taken into account as different systems will benefit from different pilot ratios. For example, a system that runs stable, constant loads will normally use a high pilot pressure, while a low, unstable load will benefit from a lower pilot pressure.
The pilot open pressure drop is a good measure of system performance and efficiency, as system pressure typically runs much higher than the pilot pressure needed to open the valve fully.
The two-stage over-centre valve
An addition to the over-centre valve family, the two-stage over-centre valve aims to tackle problems that long unstable booms suffer from, especially those with large capacity cylinders such as telescopic handlers which can suffer from instability issues.
Runaway conditions are encountered in these applications when pilot valves are opened too quickly, due to heavy loads on the cylinder. The two-stage over-centre valve uses two springs with the outer spring being affected by the pilot piston with the inner used as a pressure counterbalance, thus overcoming potential instability issues.
Which type of over-centre valve should you get?
When looking for the correct over-centre valve, you have to ensure you cater for the pressures the hydraulic unit will need to work with. In a system with high back pressure a standard over-centre valve would struggle, as the standard spring chamber is vented to the valve port through the poppet, this increases relief pressure and systems which use a closed centre directional valve would run into difficulties.
Valves are now available that help to combat this problem as the relief sections of these valves are not affected by back pressure and they are identical in every other way to a standard valve.
Finally, some over-centre valves come complete with an atmospheric venting feature, which can be a beneficial feature until they are used in a corrosive type atmosphere which could cause running problems, so it is always important to check system plans and positioning when deciding on the type of valve to go for.
Armadillo Group stock a range of over-centre valves and these can even be bought online, or call us for more info