hydraulik

Hydraulics > Gear Pumps Gear pumps are a fixed displacement pump; they are the simplest of all the pumps. Two (or multiples of) intermeshing gears are rotated forcing the oil around the outside of them. These are the most cost-effective pumps but being fixed displacement they are nearly always clutched if run from anything but a dedicated prime mover. If the drive shaft of the pump shown above was to rotate anti-clockwise then the side that we are looking at would be the suction side of the pump and the other side the pressure. The way that a gear pump works is that the gear meshing zone has no spaces in it as the teeth are meshed where as there is no such problem around the outside, as the shaft is rotated then the oil is forced around the outside of the gears, see diagram below. Vane pumps Vane pumps are often used in power steering units and automatic transmissions. The vanes are in slots in the rotor. When the rotor spins, centrifugal force pushes the vanes out to touch the casing, where they trap and propel fluid. Sometimes springs also push the vanes outward. When the vanes reach the return side they are pushed back into the rotor by the casing. Fluid escapes through a channel or groove cut into the casing. Vane pumps are more efficient and have a longer life than gear pumps but there maximum Rpm and pressure is often low. Vane pumps are available in both fixed and variable displacement Above is a basic representation of a vane pump, A and B are slots allowing access into the vane pump chamber. As the yellow section is rotated the volume in the pump chamber adjacent to A is increasing therefore sucking oil in where as that adjacent to B is decreasing therefore blowing oil out. Piston Pumps These can be in both groups: fixed or variable displacement. We rarely use fixed displacement piston pumps due to the fact that the increase in price over other forms of pump is not warranted by the duty cycle. A variable displacement piston pump will displace a volume of oil from almost zero to its maximum volume on demand. This has obvious advantages in a system where there are multiple demands of varying flows required as only the oil needed is pumped into the system. In most cases were there are multiple functions the volume of oil being pumped is controlled by the valve arrangement which needs to be set up for “Load sensing”. These pumps are designed for continuous operation so they can be direct coupled to a prime mover, though we usually clutch them when they are used for ancillary services. If they are being belt driven they will need the input shaft supported, beware of the maximum pump rpm. (Also remember there is a minimum) Piston pumps closed loop Piston pumps are the most efficient way of transmitting hydraulic power, they are used in a “Closed loop” system for efficiently transmitting large amounts of power to a single operation e.g. propulsion. Transmission performance and component life are functions of the hydraulic circuit design Circuit elements must be present to remove undesir­able heat and contamination and to control the pas­sage of cool, clean fluid into the main power loop. Other circuit elements are present to control pressure and, sometimes, to control flow. A typical dosed circuit schematic is shown above. The dosed circuit piston pump is normally fac­tory equipped with a charge pump, charge pressure relief valve, and a control. It may also be equipped with high pressure protection devices such as relief valves (shown) or pressure limiters. A loop flushing shuttle valve can be integral to the pump or motor, or may be installed externally. Other components such as the reservoir, heat exchanger, and filter are also required. The purpose of this information is to advise on generally accepted circuit ar­rangements, but it does not, and cannot, describe all possible circuits which may be appropriate for vari­ous applications. Transmission circuits are unique designs and must be developed to ensure required performance and life. Piston pumps open loop Orange = pilot pressure Dark blue = suction Red = pressure Pale blue = case pressure The pistons are attached to the main shaft and rotate with it, the yoke is static with the pump body. With a load sensing system there is a compensator mounted on the pump, there are many forms of these but the general principle is the same. A pressure signal is sent from the valve (see pvg32 control valve) back to the pump, the compensator then senses whether the pressure created on the pressure side of the pump is greater than the signal pressure from the valve, if this is so then oil is allowed into the orange section shown above, this moves a piston against the yoke which reduces the stroke of the pistons therefore reducing the amount of oil pumped per revolution. If the pump pressure is falling compared to the signal pressure then the spring takes over and increases the displacement of the pump.