GB2130685A

GB2130685A - Piston pump

Info

Publication number: GB2130685A
Application number: GB08330808A
Authority: GB
Inventors: Emile Hunsinger; Stephane Constantinidis; Jean Marye
Original assignee: RIVAPOMPE SA
Current assignee: RIVAPOMPE SA
Priority date: 1982-11-25
Filing date: 1983-11-18
Publication date: 1984-06-06
Also published as: FR2536797A1; ES8407161A1; PT77686B; PT77686A; GB8330808D0; ES527518A0; DE8333941U1; FR2536797B1; IT8368225A0

Abstract

A piston pump has a body (1) made of light metal or alloy containing a cylindrical bore (2) which is coated with at least one layer (12, 13) of hard metal oxide and, a piston (9) which slides in said bore. The piston (9) is preferably made of the same light metal or alloy as the body (1) and comprises at least one sealing ring (18) made at least partly of a plastomer. The pump may be used as a vacuum pump for the control of vehicle brakes. <IMAGE>

Description

SPECIFICATION A piston pump This invention relates to a piston pump having a cooperating piston and cylinder, suitable particularly but not exclusively, for use as a vacuum pump for controlling the brakes of a vehicle.

Vacuum pumps presently used for this type of application generally have a main body moulded from aluminium and provided with a cylindrical bore, inside which a cylindrical lining is fitted, the lining being made of steel or steel alloy and having the piston sliding in it.

This arrangement, apart from the fact that it is relatively expensive due to the fact that it is made up of two components which have to be assembled, has certain disadvantages.

Firstly it requires sealing means to be positioned beneath the main body and the lining.

This is particularly indispensible in the case of a dual action vacuum pump, to prevent the pressure difference within the two chambers of the pump from causing the air to pass from one chamber to the other by bypassing the piston. The problem is exacerbated by the fact that the coefficient of expansion of the lining is different from that of the main body, so that an intermediate volume is produced between the two components through which air can circulate.

Secondly the piston is generally made of light metal or metal alloy, which itself has a coefficient of expansion different from that of the lining. Thus when there are great fluctuations in the temperature of the pump, leakages frequently occur between the lining and the piston, or the opposite effect is experienced, of excessive wear on the piston.

The publication Olhydraulic und Pneumatik in the article "Oberflachenveredelte Alu- Zylinderrohre" volume 25, no. 12, page 929, describes a method of surface treating an aluminium tube. The process described in the publication involves anodising the tube surface so that a layer of aluminium oxide intimately bonded to the body of the tube is formed on the tube surface.

An aluminium cylinder for a jack is known from DT-OS 26 45 134, which does not have a lining.

Instead its inner surface is anodised to form a thin protective layer of aluminium oxide thereon.

However, surfaces treated by the processes described in these two known documents are subjected only to low frequency mechanical strains, which consequently cause relatively little wear. Moreover the layers formed by these processes are generally in contact with a metal piston, so that the second disadvantage mentioned above is not eliminated.

There is thus a need for a generally improved piston pump with which, for example, it may be necessary to operate at speeds up to 5,000 reciprocating movements per minute.

According to the present invention there is provided a piston pump, having a body made of light metal or metal alloy in which is a cylindrical bore with a thin layer of hard metal oxide formed electrochemically on its surface, and slidably movable in said bore, a piston fitted with an annular seal which is made at least partially of a plastomer with a low coefficient of friction and which slidably engages said bore layer.

Since the body does not have a lining, the sealing problems of conventional lined pumps automatically are eiiminated.

The layer of hard metal oxide may be formed either by anodisation or by disposition of metal oxide.

In either case two layers are really formed, namely an outer layer of hard oxide, e.g.

approximately 50 to 1 50 microns thick, and an intermediate diffusion layer, e.g. of approximately 25 microns, in which the oxide is diffused into the structure of the body metal. This intermediate diffusion layer is responsible for bonding the outer layer to the body.

The cylindrical bore in the body of the pump is thus coated with a hard layer, which may have a hardness value in the range of 300 to 450 Vickers (V.P.H.) giving high resistance to wear.

This layer is further found to have excellent friction properties, particularly with seals made of plastomer (polytetrafluoroethylene or fluocarbon compounds).

Furthermore the pump is particularly suitable for use in the field of vacuum pumps, where problems of chemical corrosion do not arise.

Naturally such a pump may further comprise ali the accessories of traditional, single or dual action pumps such as valves, control means and a cooling system. It may be lubricated with oil which may come from an engine or it may have its own lubricating means.

Thus the invention reduces wear on the piston, even at high piston displacement speeds and temperatures of up to 1 400C within the pump.

This makes it possible to obtain a vacuum pump which has a good performance and is light, at considerably reduced cost.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagrammatic axial sectional view through a pump, without a lining, according to the invention, and Figure 2 is a larger scale, partial sectional view of the pump shown in Figure 1, illustrating the layer formation on the cylinder bore and the piston/bore sealing system.

As shown in the drawings, a piston pump of the invention has a body 1 made of light metal, for example aluminium, magnesium or an alloy of those two metals not including copper and iron.

Experience has shown that a layer formed by anodisation does not adhere to the body in the presence of copper and iron. The body 1 has an internal cylindrical bore 2 closed at one side by a casing 3 and at the other by a base 4.

To form a vacuum pump the casing 3 conventionally has at least one suction valve 5, which may be connected to a suction intake of a servobrake, and a delivery valve 6.

The base 4 is fitted with a coaxial bearing bush 7, in which a rod 8 of a piston 9 slides under sealed conditions. In the case of a dual action pump it may further comprise a suction valve 10 and delivery valve 11 which may be respectively connected to the suction valve 5 and delivery valve 6 of the casing 3.

As will be seen from Figure 2, a layer is formed on the bore 2 of the main body 1 by anodisation of the body. The layer consists of an external layer 12 of alumina or magnesium oxide, which may be in the range of from 50 to 1 50 microns thick, and an intermediate diffusion layer 13, in which the alumina is included by diffusion in the crystalline structure of the aluminium body 1.

As a rule the surface state of the alumina layer 12 is good enough not to require any grinding or polishing. However, either of these operations may be considered if necessary.

The piston 9 may be in the form of a stack of two aluminium discs 14, 15 with a peripheral, facing recess 16, so as to form a peripheral groove 1 7 of square or rectangular section in the edge.

Inside the groove 17 there is a composite segment comprising an external annular seal 18 of U-shaped section, made of plastomer such as polytetrafluoroethylene or a fluocarbon compound. The concave side of the seal 18 receives or incorporates a strain bearing or spring member 19, which may be an O-ring made of elastomer or even a flexible metal O-ring.

The combination of materials used to make the sliding seal between the body 1 and the piston has been found to give an excellent sealing action, excellent friction properties and very good resistance to wear.

Claims

1. A piston pump, having a body made of light metal or metal alloy in which is a cylindrical bore with a thin layer of hard metal oxide formed electrochemically on its surface, and slidably movable in said bore, a piston fitted with an annular seal which is made at least partially of a plastomer with a low coefficient of friction and which slldably engages said bore layer.

2. A piston pump according to claim 1, wherein the piston is made of the same light metal or metal alloy as the body.

3. A piston pump according to claim 1 or claim 2, wherein the body is made of aluminium or of a light metal alloy not containing copper or iron, and wherein the layer of hard metal oxide is alumina or magnesium oxide.

4. A piston pump according to any one of claims 1 to 3, wherein the layer of hard metal oxide is obtained by anodisation or by deposition of metal oxide.

5. A piston pump according to any one of the preceding claims, in which the annular seal is made at least partially from polytetrafluoroethylene or a fluocarbon compound.

6. A piston pump according to any one of claims 1 to 5, in the form of a vacuum pump.

7. A piston pump substantially as hereinbefore described with reference to the accompanying drawings.

8. A vehicle brake system, including a piston pump according to any one of claims 1 to 7.