GB2139928A

GB2139928A - Spindle assemblies

Info

Publication number: GB2139928A
Application number: GB08407284A
Authority: GB
Inventors: David Bradford Wood
Original assignee: Cincinnati Milacron Inc
Current assignee: Milacron Inc
Priority date: 1983-03-21
Filing date: 1984-03-21
Publication date: 1984-11-21
Also published as: DE3408964C2; FR2543032B1; JPS59175901A; DE3408964A1; GB8407284D0; GB2139928B; FR2543032A1

Abstract

A rotatable spindle is carried in bearings (17) located in one end of a bore in a spindle support (20), the other end of the support being fixed to a housing of the machine so as to cantilever the support. A damping chamber (32) is formed lengthwise of the support between the machine housing and the support, and a damping medium is provided in the damping chamber to reduce vibrations of the cantilever assembly. In one embodiment, a viscous damping fluid is utilised to effect squeeze film damping, and in an alternative embodiment a visco-elastic damping medium is utilised. <IMAGE>

Description

SPECIFICATION Improvements relating to spindle assemblies In workholding spindle assemblies typically used in turning and grinding machines, where the work is held in a chuck attached to the nose of a spindle, the most common prior art assemblies teach that the spindle mounting bearings should be placed as close to the end of the spindle as is possible, and that the outer race of an antifriction bearing assembly should be supported by a rigid machine structure, such as a spindle housing assembly backed up by ribs orwallsto give a stiff structure with a minimum of deflection at the end of the spindle. The basic teaching of these assemblies is to produce a high static stiffness by the inherent mechanical springs involved, but not thought is given to damping the structure from induced vibrations, other than the material damping properties such as are found in nodular iron.

Present day assemblies are utilising increasingly more units made from welded steel structures, lacking cast iron damping properties, and thus the chance of vibrations causing problems at the worktool interface are becoming more acute.

According to this invention, there is provided a spindle assembly comprising a housing, a spindle rotatably mounted in the housing, a spindle support secured to the housing and a bearing assembly operative between the spindle and a bearing portion of the spindle support, wherein a damping chamber is provided between the spindle support and the housing, within which a damping medium is or may be provided.

Thus conveniently the spindle support may be secured to the housing in a manner which permits limited flexing movement between the spindle support and the housing in directions radially of the spindle.

Thus in the use of the invention, the damping chamber is filled with a viscous medium such as heavy machine oil, or alternatively a visco-elastic damping medium, so that a thin viscous film is provided in the damping chamber. In operation, the spindle is free to move slightly because of the cantilever type mounting of the spindle support under the influence of machine vibrations, but the vibrations are damped by the phenomenon known as squeeze film damping, wherein the relatively sticky oil operating on opposing surfaces of the damping chamber will serve to attenuate the vibrations.

Preferably the spindle support is elongate axially of the spindle, and comprises a mounting portion interfitted axially with the housing or a part secured thereto, the damping chamber advantageously being located, in the axial direction, between the mounting portion and the bearing portion.

Preferably the damping chamber is defined in paart at least by adjacent cylindrical surfaces of the spindle support and the housing, or a part secured thereto.

Where the damping medium is a viscous fluid, preferably means is provided to seal the damping chamber to prevent the escape of damping fluid.

Alternatively, the damping medium may be provided by a visco-elastic damping medium.

According to this invention there is also provided a method of damping vibrations in a spindle assembly comprising the following steps: (a) providing a rotary bearing at each end portion of a spindle; (b) mounting one end of an elongate spindle adaptor to one of said rotary bearings with said adaptor extending towards the other bearing; (c) fixing the opposite end of the adaptor to the machine housing; (d) supporting the other of said rotary bearings in said housing; (e) providing a damping cavity generally between the ends of the spindle adaptor, said damping cavity being bounded in part by a damping surface of the spindle adaptor;and (f) providing a damping medium in said damping cavity.

There will now be given a detailed description, to be read with reference to the accompanying drawing, of a spindle assembly which is a preferred embodiment of this invention, having been selected for the purposes of illustrating the invention by way of example.

The accompanying drawing is an axial crosssection cut through a machine tool head stock, showing the spindle assembly which is the preferred embodiment of this invention.

The spindle assembly 10 which is the preferred embodiment of this invention comprises a typical machine headstock 11 and a spindle 12 having a spindle nose 13 at its outboard end for carrying a workholding chuck 14. The spindle 12 is provided with a spindle bearing diameter 15 which is received in the inner race 16 of a double rowantifriction bearing 17. The antifriction bearing 17 has an outer race 18 which is relatively stationary in the assembly 10 while the inner race 16 rotates with the spindle 12.

The outer race 18 of the bearing 17 is carried in a bearing bore 19 of a supportforthe spindle, afforded by a spindle adaptor 20 which is generally cylindrical; having an outer flange 21 for securing the adaptor 20 into a machine housing 22. Elongated cap screws 23 are provided through the adaptor flange 21 and are received into threaded holes 24 lying at the bottom of a clearance hole 25 in the machine housing 22, so that some relative lateral movement of the screws 23 can be accommodated on movement of the spindle adaptor 20 under the influence of machine vibration.A reduced diameter 26 on the spindle adaptor 20 extends into a co-operating cavity 27 machined in the machine base 22, and at the end 28 of the cavity 27, a spindle adaptor mounting diameter 29 is tightly fitted into a bore 30 in the machine base 22 so that the spindle adaptor 20 creates a cantilever-type mounting arrangement of the spindle nose 13 with respect to the press-fit diameter 29 on the spindle adaptor 20. A pair of seals 31 are provided at spaced apart points in the damping cavity 27, so that a closed damping chamber 32 is therein formed. An access hole 33 is provided through the wall 34 of the machine base 22, so that a viscous medium 35 such as hydraulic oil may be introduced into the damping chamber 32, and the chamber 32 may be filled. The access hole 33 is plugged after filling.

The outboard surface 36 of the spindle adaptor 20 carries a seal mounting cap 37, which has a seal 38 surrounding the spindle sal diameter 39, so that any internal lubrication which is provided to the antifriction bearing 17 will not be lost. The rear end 40 of the spindle 12 is conveniently supported in an antifriction bearing 41, which is supported and backed up directly by a bearing adaptor 42 and the rear wall 43 of the machine base 22.

Thus it can be seen that the rear end 40 of the spindle 12 is conventionally supported in a rigid structure having a high static stiffness, while the front of the spindle 12 is supported in a manner tending to produce a low static stiffness, that is, when deflection of the spindle nose 13 is statically measured. This type of mounting has not been seen in the prior art, since it tends to present a mounting arrangement which is seemingly the antithesis of good design practice.However, in actuality, when the damping chamber 32 is filled with a viscous medium 35 and the adaptor 20 is allowed to relatively move with respect to the machine base 22, i.e. in a deflectable manner under the infludence of machine vibrations and at high vibratory rates, the vibrations are attenuated by the phenomenon known as squeeze film damping, wherein the relatively shiftable members 20, 22 are constantly compressing and extending the tacky damping fluid 35. The overall net effect is that a high dynamic stiffness is achieved, that is, while the spindle 12 is rotating and cutting is being performed.

The phenomenon of film squeeze damping occurs particularly in chambers having a thickness (in the preferred embodiment in the radial direction) of less than 0.025 inches, but is particularly effective in chambers having a thickness in the order of 0.005 inches or less.

As an alternatively embodiment, a visco-elastic damping material, such as PVC (polyvinyl chloride) may be substituted in the damping chamber 32 for the viscous medium 35. This type of material has the characteristic of being able to dissipate energy when displaced. The visco-elastic materials used in damping units behave quite definitely in an elastic manner up to some stress limit, but once that stress is exceeded, it flows until its area increases and stresses do down until it becomes elastic again.

Strictly considered, in a damper designed for squeeze film, the direct substitution of a visco-elastic material would result in somewhat less damping of the structure, but this may be suitable for some applications. the visco-elastic materials can have a damping capability twenty times that of rubber, but the squeeze film damper can possess a damping capability three or four times that of the visco-elastic damper.

Claims

1. A spindle assembly comprising a housing, a spindle rotatably mounted in the housing, a spindle support secured to the housing and a bearing assembly operative between the spindle and a bearing portion of the spindle support, wherein a damping chamber is provided between the spindle support and the housing, within which a damping medium is or may be provided.

2. A spindle assembly according to Claim 1 wherein the spindle support is secured to the housing in a mannerwhich permitsjimitedflexing movement between the spindle support and the housing in directions radially of the spindle, damping medium in the damping chamber being operative to damp such movement.

3. A spindle assembly according to one of Claims 1 and 2 wherein the spindle support is elongate exially of the spindle, and comprises a mounting portion interfitted axially with the housing or a part secured thereto.

4. A spindle assembly according to any one of the preceding claims wherein the damping chamber is located, in the axial direction, between the mounting portion and the bearing portion.

5. A spindle assembly according to any one of the preceding claims wherein the damping chamber is defined in part by adjacent cylindrical surfaces of the spindle support and the housing, or a part secured thereto.

6. A spindle assembly according to any one of the preceding claims wherein the depth of the damping chamber in a direction radially of the spindle is less than 0.025 inches.

7. A spindle assembly according to any one of the preceding claims comprising sealing means to prevent the escape of damping medium from the damping chamber.

8. A spindle assembly according to any one of the preceding claims werein the damping chamber contains or is adapted to contain a damping medium afforded by a viscous fluid.

9. A spindle assembly according to any one of Claims 1 to 6 wherein the damping chamber contains or is adapted to contain a visco-elastic damping medium.

10. A method of damping vibrations in a spindle assembly comprising the following steps: (a) providing a rotary bearing at each end portion of a spindle; (b) mounting one end of an elongate spindle adaptorto one of said rotary bearings with said adaptor extending towards the other bearing; (c) fixing the opposite end of the adaptor to the machine housing; (d) supporting the other of said rotary bearings in said housing; (e) providing a damping cavity generally between the ends of the spindle adaptor, said damping cavity being bounded in part by a damping surface of the spindle adaptor; and (f) providing a damping medium in said damping cavity.

11. A method according to Claim 10 wherein the step (f) includes providing a viscous damping medium in said damping cavity, and further includes the step of sealing the damping cavity.

12. A method according to Claim 10 wherein the step (f) includes providing a visco-elastic damping medium in said damping cavity.

13. A spindle assembly constructed and arranged substantially as hereinbefore described with reference to the accompanying drawing.

14. Any novel feature or novel combination of features disclosed herein and/or shown in the accompanying drawing.