SAFARI

28 POWER STEERING Vane Pump : XII Maintenance instruction and functional description I. Maintenance and oils : 1. Checks: The pump is lubricated by the hydraulic oil, meaning that no additional servicing of the bearings is necessary. On the other hand, it is important that the oil level in the tank be checked at regular intervals. We recommended that this check be carried out at least every 5000 kilometers travelled. The possibility of air being trapped at various points within the oil circuit and the fact that very small air bubbles still remain in the oil even after air bleeding make it necessary for the oil level to be checked while the pump is running. The trapped air is then under operation pressure, subsequently expanding under normal atmospheric pressure when the pump has been shut down. Experience has shown that this causes the oil level to rise approximately 1 to 2 cm in the oil tank when the pump has been shut down. It is recommended to check the oil level in the oil tank prior to starting the pump and, if necessary, to top up the oil level until it is approximately 2 cm above the upper mark when the pump is at a standstill. Only then should the pump be started. If the oil level then drops excessively (in other words more than 1 to 2 cm), the entire hydraulic system must be bled. Refer to the section “Oil filling and air bleeding “ In order to prevent the intake of air, all screw connections and pump lines should be checked for tightness and, if necessary, retightened when the vehicle involved is in for general servicing. 2. Filter change: Normally after every 72,000 Kms/43000 miles running of the vehicle the filter is to be changed. The filter in the oil tank should also be changed when repairs are performed on the pump or the steering unit and during the main steering unit inspection (refer to the inspection instructions II - 6.1). 3. Steering oil: Normally oil is to be changed at every 80,000 Kms/ 48000 miles running. A suitable hydraulic oil must be used in order to ensure perfect functioning of the pump.The hydraulic oil also lubricates the pump, meaning that only one type of oil is required for the entire system. The unit should be filled with suitable ATF oils, having a viscosity of approximately 26mm 2 /s (cSt) at 50 degree, a setting point below –35 degree and having a low foaming tendency. Oils of higher viscosity may result in an excessive under pressure in the inlet line, thus possibly causing pump noise and destruction. Some of the types of oil recommended by us for the first filling of pumps are listed on the Oil –Tank. II. Functional description : ZF vane pumps have been specifically designed to supply oil to hydraulic- assisted steering systems.The low number of moving parts (drive shaft, rotor and vanes) and the minimum possible face – end play of these elements with respect to the drive – side and cover – side faceplate ensure high efficiency and, consequently, the best possible utilization of power. This efficiency is also partially the result of the pairs of two opposite suction and pressure lines, a design whereby the ten pump cells each convey twice their own cell volume per revolution.The double arrangement of the suction and pressure lines results in mutual cancellation of the hydraulic radial forces acting upon the rotor. When turning the drive shaft and the rotor connected to it, the vanes guided the ring are also driven. Centrifugal force and pressure oil which can pass via bores and grooves from the pressure chamber on the inside faces of the rotor, force the vanes against the track of the camring. The pump cells are thus separated from each other, causing intake as the volume rises and discharge as the volume decreases. The oil passes from the two pressure chambers into the space between the cover and the cover – side faceplate, which is thus forced against the camring at operation pressure, also to the flow – limiting valve, and via throttle to the discharge line. An increase speed and or increase flow results in an increasing pressure drop downstream of the choke and, consequently, on the spring side of the valve