Radial piston pumps have been extensively used in motor vehicles for transporting lubricating oil, pumping fuel, and as pressure generating means for hydraulically operated servomechanisms. Such pumps find further use as hydraulic pumps for power steering, shock absorbers, clutches and continuous transmissions, automatically controlled transmissions, and hydraulically operated driving and auxiliary equipment, and for operational machines and the like. Radial piston pumps are predominately installed in cases where a higher hydraulic pressure level is necessary.
Serving as pumps of the displacement type, radial piston pumps do not deliver a pumped medium in continuous flow, but irregularly, in partial volumes per revolution of a driven eccentric. The cyclically transported volumes give rise to pressure variations and pulsations, both at the intake and output ports of the pump. The said pulsations inlet and outlet overlap, due to the opening and closing of the of the pump chambers, that is to say, the cylinders. The impacts are particularly severe if, during operation with volute spring activated inlet and outlet valves, suddenly spaces are made available which exhibit large pressure differentials. Beyond this, as rule, large pressure swings also occur, if a system operates at high pressure, or if a cylinder is partially filled.
If pressure in a cylinder attains an opening pressure of the annular volute spring of a valve, then the valve lifts away from its seat, and the hydraulic fluid, for example pressurized oil, is pushed into a sump. If the pressure in the cylinder falls below the closure pressure of the spring loaded valve, then this valve impacts once again on the seat and causes thereby a loud hammering noise. This performance repeats itself at every rotation of the driven eccentric, in accord with the number of piston-cylinder combinations of a pump.
The noise is just that much louder, as the opening and closure process becomes more dynamic. Also influencing the said hammering noise are the related opening pressures and closing pressures and as well, the rate of increase of pressure at the instant of opening generates noise. If these values are very high, then the spring loaded valve will be lifted instantaneously very far from its seat and accordingly return to its seated position with considerable force. The pressure impacts of all pistons produce a general noise, which resounding from the body of the pump, radiates as audible air-borne noise.
In order to both reduce and mitigate the peaks of the pressure impacts, and also to reduce the noise generation of the radial piston pump, there is proposed in DE A 43 36 673, a radial piston pump, which has a plurality of pistons set into corresponding cylinder borings in a pump housing, wherein each piston is loaded by a spring, which spring abuts against a detent. The drive shaft is axially affixed to an eccentric, upon which a slide bushing is placed. Between one inner slip ring, which is pushed onto the said slide bushing, and a concentric outer slip ring, is located a damping element, which, for example, is designed as a flat, compression spring. Upon rotation of the eccentric, in this way, the respective piston which is expelling oil under pressure can, to some extent, act resiliently against the assigned section of the slip ring, so that the pressure spike normally occurring at the beginning of the pressure thrust can be reduced in intensity.
In another published embodiment, the damping element possesses the shape of slotted annular spring, wherein, equally distributed projections supportingly oppose one another across the inner and outer diameters of said annular spring. The supporting projections permit sufficient clearance between them, so that the particular piston making the thrust can resiliently modify itself.
In yet another embodiment an elastic ring is inserted between the two slip rings. The said elastic ring can well be made of rubber and be vulcanized onto both sides of the slip ring. Instead of a rubber ring, this disclosure also allows that, between the inner and the outer slip rings, an annular ring may be inserted, which is again vulcanized, but consists of a combination of multiple straight sections.
In DE A 101 26 151 a slip ring for a radial piston pump is described, which consists of an inner ring and a thereto coaxially arranged outer ring, between which a damping element is interposed. The damping element is constructed as being of “one piece” and has on both sides, respectively, a bulged rim, which lies against the side rim of the inner ring against the outer circumference thereof, and at the rim of the outer ring within the inner circumference thereof, whereby, between the two said bulged rims, a connecting structure is provided, which, for example, can be formed by an additional damping ring, which is connected with the said bulged rim by means of fabricated webs. In addition, it is possible, that by appropriate formation of the damping element between the inner ring and the outer ring, chambers are created, into which a filling fluid may be introduced, so that the rigidity of the slip rings can be made variable.
To this purpose, along the outer circumference of the damping element between the edge bulges, a plurality of chambers are provided between the inner and the outer ring. The individual said chambers possess an inlet port for the liquid to be drawn in, the said liquid being, for example, oil, so that when the said oil is to be drawn in at low temperatures, by means of filling the chambers of the slip rings, the elasticity thereof is diminished and thereby, by means of the eccentricity of the eccentric a defined piston thrust cannot, or nearly cannot, be affected by the elasticity of the slip rings.
Also disclosed in this publication, in order to reduce noise from radial piston pumps, elastomers are interposed as a layer between the driven eccentric and the pistons, which latter are arranged in a star shape. This layer insertion is made in particular in the form of elastomer slip rings, which are inlaid between the driven eccentric and an outer slip ring. The said elastomer slip ring can be put in place by simply being laid in position, or by being impressed, or by vulcanization onto a contiguous part. First, the damping characteristic of these rings, which, for instance, generates itself from the elasticity of their construction, reduces the pressure increase gradient in an individual cylinder, which is the cause for the objectionable noise and pulsation development. Second, that property of reducing in the transmission, both the noise intensity and its radiation from the pump body, is improved by the insertion of the said elastomer components.
The conventional slip ring assembly for radial piston pumps, as it is applied generally for a continuous and automatic transmission, i.e., the so-called CFT-transmission, still exhibits the disadvantage of considerable weight, so that this causes an imbalance in the gear train and leads to vibrations of the transmission.