GB2270642A

GB2270642A - Cylinder for rotary positive dis placement machine

Info

Publication number: GB2270642A
Application number: GB9219060A
Authority: GB
Inventors: Derek Arthur Lansdell
Original assignee: Desoutter Ltd
Current assignee: Air Compressors and Tools Ltd
Priority date: 1992-09-09
Filing date: 1992-09-09
Publication date: 1994-03-23
Also published as: JPH079053A; GB9219060D0

Abstract

A cylinder (6) is made by rolling a sheet metal blank (1) to form a cylindrical tube. The blank has a polished surface (2) which forms the inner surface of the tube, a pair of opposite parallel edges (3) which form tube ends normal to the axis, and a pair of opposite terminal edges (4) which together form a slot (7) which intersects the tube ends and which extends obliquely to the tube axis over at least a major part of the length of the tube. The cylinder (6) is mounted in an elastically compressed state in a bore in a casing of a rotary positive displacement machine having a vaned rotor. The slot (7) may be open, to serve as an inlet or exhaust opening, or may be closed. <IMAGE>

Description

Cylinder for Rotary Positive Displacement Machine This invention relates to cylinders for rotary positive displacement machines having vaned rotors, in particular (but not solely) vaned air motors.

Such cylinders are usually made from sintered or extruded tubes, and a burnishing ball has to be passed through the cylinder in order to achieve a polished inner surface suitable for interaction with the rotor vanes.

It would be desirable to be able to produce a cylinder without the need for ball burnishing of the cylinder.

The present invention provides a method of making a cylinder for a rotary positive displacement machine having a vaned rotor, the method comprising rolling a sheet metal blank to form a cylindrical tube, the blank having a polished surface which forms the inner surface of the tube, a pair of opposite parallel edges which form tube ends normal to the axis of the tube, and a pair of opposite terminal edges which together form a slot which intersects the tube ends and which extends obliquely to the axis of the tube over at least the major part of the length of the tube.

The invention also provides a cylinder comprising a cylindrical tube formed from a sheet metal blank, the cylinder having a slot which intersects the ends of the tube and which extends obliquely to the axis of the tube over at least the major part of the length of the tube.

The invention also provides a rotary positive displacement machine comprising the said cylinder and a vaned rotor therein, the cylinder being accommodated in and supported by a bore in a casing of the machine.

In one embodiment, when mounted in the casing, the slot may be open and may serve as an exhaust or inlet opening which is overlapped by a recessed cavity in the casing.

In other embodiment, when mounted in the casing, the slot may be closed, with the opposite edges of the slot abutting against each other. In this case, one or more openings may be provided in the blank to serve as an inlet and/or an outlet.

Preferably the slot is helical over at least the major part of the length of the tube. However, at least one end portion of the slot may extend substantially axially, and this end portion may occupy, for example, up to 10% of the length of the tube.

The slot preferably extends helically at an angle of at least 200 (more preferably at least 300) and at most 700 (more preferably at most 600) to the axis of the tube.

The circumferential extent of the slot, when it is to serve as an exhaust (or inlet) opening, is preferably at least 1200, more preferably at least 140 , and is preferably at most 1800, more preferably at most 1500. On the other hand, if the slot is to be closed, its circumferential extent is preferably less than 40 , more preferably at most 300.

The blank may be stamped from sheet metal, which will usually be provided with a polished surface in the course of its manufacture, but which may be subjected to a polishing step before or after stamping, if desired. The roughness of the polished surface is preferably less than 20 ym CLA (centre line average), more preferably 16 ym or less, most preferably 10 to 15 ym.

The blank may be of steel, preferably spring steel, and may have a thickness of, for example, 4 mm, 3.5 mm, 3 mm, or less, preferably at least 1.5mm. If the rolled tube is sufficiently elastic, it can be self-supporting in the bore in the casing, without having to be bonded (e.g. welded) to the casing.

The invention will be described further, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a sheet metal blank for making a cylinder for an air motor having a vaned rotor; Figure 2 is a side view of the cylinder; Figure 3 is an end view of the cylinder, taken in the direction of the arrow III in Figure 2; Figure 4 is a diagrammatic axial section through the air motor; Figure 5 is a side view of another cylinder for an air motor having a vaned rotor; and Figure 6 is an end view taken in the direction of the arrow VI in Figure 4.

The blank 1 shown in Figure 1 has been stamped from EN42 spring steel strip 2.6 mm thick, having a polished surface 2 whose roughness, as supplied, is 10-15 ym CLA. If necessary, the edges of the blank are smoothed after stamping. The blank has a pair of opposite parallel edges 3 whose length is 151 mm. The overall length of the blank is 211 mm and its width is 30 mm.

The terminal edges 4 of the blank have end portions 4a which are 3 mm long and are normal to the long edges 3, but over most of their length the terminal edges 4 extend at an angle of 210 to the edges 3.

The blank 1 is rolled (bent) around a cylindrical former (not shown), with the polished surface 2 in the inside, causing plastic deformation of the blank (beyond its elastic limit) and bringing the opposite edges 4 substantially into mutual abutment.

When the resulting cylindrical tube or cylinder 6 is taken off the former, the terminal edges 4 separate, owing to the residual elasticity of the spring steel, so that a spiral slot 7 with an angular circumferential extent ol of 1410 is defined between the edges 4. The external diameter of the cylinder 6 is 54 mm.

The cylinder 6 is elastically compressed and is fitted in a bore 8 in a casing 9 of the air motor. A vaned rotor 11 is mounted in the cylinder 6. The vanes (not shown) run smoothly over the oblique edges 4 of the cylinder 6. Compressed air is supplied to one end of the cylinder 6 through a port 12 and is exhausted through the slot 7 into an annular recessed cavity or gallery 13 in the casing 9.

The cylinder 6' shown in Figures 5 and 6 is again produced by rolling a blank (not shown) around a cylindrical former. The blank is stamped from EN42 spring steel strip at least 1.5mm thick, having a polished surface 2 with a roughness of 10-15 ym CLA. Inlet and exhaust openings (not shown) are formed in the blank by stamping prior to rolling.

The cylinder 6' has a length of 30mm and an internal diameter of 49mm when a spiral slot 7' defined by oblique terminal edges 4' is closed, i.e. when the edges 4' abut. The slot 7' is at about 300 to the cylinder axis and has an angular circumferential extent oW' of 380. The slot 7' is closed when the cylinder 6' is elastically compressed and fitted in a bore in a casing (not shown) of the air motor. The inlet and exhaust openings (not shown) are overlapped by corresponding recesses in the casing. The casing supports the cylinder over one half of its length, for example.

Claims

Claims:

1. A method of making a cylinder for a rotary positive displacement machine having a vaned rotor, the method comprising rolling a sheet metal blank to form a cylindrical tube, the blank having a polished surface which forms the inner surface of the tube, a pair of opposite parallel edges which form tube ends normal-to the axis of the tube, and a pair of opposite terminal edges which together form a slot which intersects the tube ends and which extends obliquely to the axis of the tube over at least the major part of the length of the tube.

2. A method as claimed in claim 1, in which the blank is made of spring steel.

3. A method as claimed in claim 1 or 2, in which the blank has a thickness of at least 1.5 mm.

4. A method as claimed in claim 3, in which the blank has a thickness of at most 4 mm.

5. A method as claimed in any of claims 1 to 4, including the preliminary step of stamping the blank from sheet metal.

6. A method as claimed in claim 5, including the step of polishing the blank after stamping.

7. A method as claimed in claim 5 or 6, in which the stamping step forms at least one opening in the blank.

8. A method as claimed in any of claims 1 to 7, in which the slot is helical over at least the major part of the length of the tube.

9. A method as claimed in claim 8, in which the slot extends helically at an angle of at least 200 to the axis of the tube.

10. A method as claimed in claim 8 or 9, in which the slot extends helically at an angle of at most 700 to the axis of the tube.

11. A method as claimed in any of claims 1 to 10, further comprising the step of mounting the tube in a bore in a casing of the machine.

12. A method as claimed in claim 11, in which the slot is open and is overlapped by a recessed cavity in the casing.

13. A method as claimed in claim 12, in which the slot has a circumferential extent in the range from 1200 to 1800.

14. A method as claimed in claim 11, in which the slot is closed.

15. A method as claimed in claim 14, in which the slot has a circumferential extent of less than 400.

16. A method as claimed in any of claims 11 to 15, in which the tube is in an elastically compressed state in the bore.

17. A cylinder for a rotary positive displacement machine having a vaned rotor, the cylinder comprising a cylindrical tube formed from a sheet metal blank, the cylinder having a slot which intersects the ends of the tube and which extends obliquely to the axis of the tube over at least the major part of the length of the tube.

18. A cylinder as claimed in claim 17, in which the tube has a polished internal surface.

19. A cylinder as claimed in claim 18, in which the polished internal surface has a roughness of less than 20 Xm CLA.

20. A rotary positive displacement machine comprising a casing having a bore, a cylinder accommodated in and supported by the bore, and a vaned rotor in the cylinder, the cylinder comprising a cylindrical tube formed from a sheet metal blank, the cylinder having a slot which intersects the ends of the tube and which extends obliquely to the axis of the tube over at least the major part of the length of the tube.

21. A rotary positive displacement machine including a vaned rotor and a cylinder produced by a method according to any of claims 1 to 16.

22. A method of making a cylinder for a rotary positive displacement machine having a vaned rotor, substantially as described with reference to Figures 1 to 3 or Figures 5 and 6 of the accompanying drawings.

23. A cylinder for a rotary positive displacement machine having a vaned rotor, substantially as described with reference to, and as shown in, Figures 2 and 3 or Figures 5 and 6 of the accompanying drawings.

24. A rotary positive displacement machine substantially as described with reference to Figures 2 to 4 or Figures 5 and 6 of the accompanying drawings.