GB2279968A

GB2279968A - Suspension unit with damped second spring

Info

Publication number: GB2279968A
Application number: GB9312517A
Authority: GB
Inventors: Grahame Russell Key
Original assignee: Hoover Ltd
Current assignee: Hoover Ltd
Priority date: 1993-06-17
Filing date: 1993-06-17
Publication date: 1995-01-18
Anticipated expiration: 2013-06-17
Also published as: GB2279968B; GB9312517D0

Abstract

A suspension unit, particularly for the tub and drum assembly of a washing machine, comprises first and second springs (28, 31) operatively arranged between two opposed mounting elements (24, 29) of the unit, and a damper (e.g. piston 30) disposed between the two springs. Under small amplitude loading oscillations, the first spring (28) deflects cyclically without movement of the damper, whilst under larger amplitude loading oscillations (experienced particularly during spin drying), the second spring (31) also deflects cyclically but under damping by frictional displacement of the damper. The damper may be in the form of a bush (56, figure 7) slidable on a rod (50). <IMAGE>

Description

A Suspension Unit This invention relates to a suspension unit and more particularly but not solely to a suspension unit for the tub and drum assembly of a washing machine.

A washing machine generally comprises a tub and drum assembly which includes a tub and, rotatably-mounted within the tub, a drum to receive clothes or other articles to be washed.

During the machine's operating cycle, the drum is rotated at high speed to expel water centrifugally from the clothes etc.

During acceleration in this so-called spin phase, when the clothes etc. are forced against the cylindrical wall of the rotating drum, an out-of-balance condition often arises due to the clothes becoming unevenly distributed, causing vibration of the drum as it rotates. In order to prevent these vibrations being transmitted to the cabinet of the machine, normally the tub and drum assembly is mounted resiliently.

However, at some spin speeds, particularly a range of speeds as the drum accelerates but prior to the drum reaching its top speed, the tub and drum assembly tends to oscillate with large amplitude: in order to restrict the amplitude of oscillation, the suspension system normally includes damping. At or near peak spin speeds, the amplitude of oscillation becomes small again and the damping is undesirable and contributes to the transmission of vibrations to the machine cabinet.

It is common for the suspension system of washing machine tub and drum assemblies to comprise one or more suspension units which provide both resilience and damping.

Suspension units have been proposed which provide damping for large amplitudes of oscillation, but little or no damping for small amplitudes of oscillation. DE-3616373 discloses one such suspension unit, comprising a piston which slides frictionally within a tube of non-uniform diameter.

Normally the piston lies in a large-diameter portion of the tube, allowing oscillations of small amplitude to take place with little or no damping. Under large amplitude oscillations, the piston slides into smaller-diameter portions of the tube either side of the large diameter portion, producing significant frictional engagement and therefore damping.

However, the rest position of the piston varies depending upon the weight of the load within the drum, so that an asymmetric damping effect is produced unless the load corresponds to a predetermined design value.

GB-2078882 and US-4854559 disclose suspension units having a piston which is free to move, without damping, for oscillations of less than a predetermined amplitude, but which hits stops, to bring frictional damping into effect, under oscillations of greater amplitude. These suspension units operate independently of the weight of the load in the drum, but they can generate noise as the piston repeatedly hits the stops.

In accordance with this invention, there is provided a suspension unit which comprises two opposed mounting elements, first and second spring elements operatively arranged between said mounting elements, and damping means for damping cyclical deflections of said second spring element, and arranged so that under small amplitude loading oscillations the first spring element deflects cyclically independently of said damping means and under larger amplitude loading oscillations cyclical deflections of the second spring element are damped by said damping means.

The suspension unit may be arranged so that the cyclical deflections of the first spring element are also damped: preferably the degree of damping provided for the first spring element is small compared with that provided for the second spring element in most, but not all circumstances.

Preferably the two spring elements are disposed in series between the two mounting elements. Preferably the damping means comprises an element disposed between the two spring elements and arranged for frictional sliding movement.

In one embodiment, the suspension unit may comprise a tube in which the two spring elements are disposed, the damping means being a frictional piston positioned between the two spring elements.

In another embodiment, the suspension unit may comprise an elongate element carrying the two spring elements, the damping means being a member frictionally slidable along that elongate element.

Also in accordance with this invention, there is provided a suspension unit which comprises two opposed mounting elements, first and second spring elements operatively arranged between said mounting elements and a member disposed between said first and second spring elements and arranged for frictional displacement relative to one of said mounting elements.

The spring elements preferably comprise coil springs but they may instead comprise leaf springs or bodies of resilient material such as rubber or foamed plastics. The spring elements may comprise different portions of a common spring or resilient body.

Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings, in which: FIGURE 1 is a sectional view of a conventional suspension unit for a washing machine tub and drum assembly; FIGURE 2 is a schematic diagram showing a washing machine tub and drum assembly supported by suspension units as shown in Figure 1; FIGURES 3a and 3b are graphs of force-againstdisplacement for the suspension unit of Figure 1; FIGURE 4 is a sectional view of a first embodiment of suspension unit in accordance with this invention; FIGURE 5 is a schematic diagram showing a washing machine tub and drum assembly supported by suspension units as shown in Figure 4; FIGURES 6a and 6b are graphs of force-againstdisplacement for the suspension unit of Figure 4; and FIGURE 7 is a sectional view through a second embodiment of suspension unit in accordance with this invention.

Referring to Figure 1 of the drawings, there is shown a conventional suspension unit for a washing machine tub and drum assembly, the suspension unit comprising a piston 6 frictionally slidable in a tube 1. A mounting shaft 2 projects from a closed end of tube 1 and the piston 6 is mounted to a rod 4 which extends through a fitting 3 in the opposite end of the tube 1. A coil spring 8 is disposed inside the tube 1 between the piston 6 and the closed end of the tube. As shown diagrammatically in Figure 2, a washing machine tub and drum assembly 10 is usually supported by two or more such suspension units, having their mounting shafts 2 fixed to a chassis 7 of the machine and their piston rods 4 fixed to the tub and drum assembly 10. The springs 8 of the suspension units provide resilient support for the tub and drum assembly 10, to inhibit vibrations being transmitted to the chassis of the machine 7, and the frictional engagement of the pistons 6 with the inner surface of their tubes 1 provides damping.

Figure 3a shows a graph of force-against-displacement for the suspension unit of Figure 1 when subjected to low amplitude oscillations: m represents the static value of applied force (the axial force acting on the suspension unit under static loading), and the corresponding mid-value of static displacement of the piston. When the piston has moved in one direction to one end of its stroke, for example when it arrives at point a in Figure 3a, the piston does not start to move in the opposite direction until point b, when the force acting on the piston 6 has developed sufficiently to overcome its frictional engagement with the tube 1. The piston 6 then moves with deflection of the spring 8, as indicated by the line b-c: the slope of this line is governed by the spring rate of the spring 8. Similarly when the piston 6 reaches the opposite end of its stroke (point c), it does not start to move towards point a again until point d, when the return force on the piston has developed sufficiently to overcome its frictional engagement with the tube 1: the piston 6 then returns to point a with deflection of the spring 8. It will be appreciated that the curve abcd represents a hysteresis loop, the area within the loop representing the mechanical energy which is mainly converted to heat due to friction. Figure 3b shows a similar graph of force-against-displacement for the suspension unit when subjected to large amplitude oscillations.

Referring to Figure 4, there is shown an embodiment of suspension unit in accordance with this invention, comprising a piston 30 frictionally slidable in a tube 21 having a mounting shaft 29 projecting from a closed end thereof. A piston 25 is slidable within the tube 21 and is mounted to a rod 24 which projects through a fitting 23 in the opposite end of the tube 21. A first coil spring 28 is disposed between the pistons 25 and 30, and a second coil spring 31 is disposed between the piston 30 and the closed end of the tube.

Figure 5 shows diagrammatically a washing machine tub and drum assembly 10 supported by two of the suspension units shown in Figure 4 (although in practice there may be more than two such units), with their mounting shafts 29 fixed to the chassis 7 of the machine and their piston rods 24 fixed to the tub and drum assembly.

Referring to Figure 6a, which is analogous to Figure 3a, under low amplitude oscillations of the tub and drum assembly 10 shown in Figure 5, the changes in load applied to each suspension unit are not large enough to overcome the friction between the damping piston 30 and the tube 21: the first coil spring 28 deflects cyclically to provide a resilient support for the tub and drum assembly 10 as the piston 25 oscillates between points a' and b', the slope of the line in Figure 6a depending upon the spring rate of the spring 28.

Referring to Figure 6b, under larger amplitude oscillations of the tub and drum assembly 10, the changes in applied load are greater and at times the damping piston 30 is subjected to forces sufficient to overcome its frictional engagement with the tube 21. For example, when the piston 25 reaches one end of its stroke (point a') and then moves in the opposite direction, firstly it moves to point b' under deflection of the spring 28 alone, then the piston 30 starts to move with damped deflection of spring 31 so that piston 25 moves to point c', being the opposite end of its stroke. Now the piston 25 reverses and moves to point d' under deflection of spring 28 alone, at which point piston 30 starts to move with damped deflection of piston 30 so that piston 25 returns to point a'.

It will be appreciated that under small amplitude oscillations of the tub and drum assembly 10, only piston 25 reciprocates (whilst piston 30 and spring 31 remain static), and the oscillations of the tub and drum assembly are undamped. Under large amplitude oscillations, both first and second springs 28,31 deflect cyclically, but the cyclical deflections of spring 31 are frictionally damped by the piston 30.

Referring to Figure 7, there is shown a second embodiment of suspension unit in accordance with this invention, comprising a rod 50 attached at one end to a bracket 52 fixed to the tub and drum assembly, and extending at its other end through an aperture in a guide 53 mounted on a bracket fixed to the chassis of the washing machine. First and second coil springs 54,55 are disposed co-axially on the rod 50. A damping bush 56 is frictionally slidable on the rod 50: the first coil spring 54 is disposed between the damping bush 56 and the guide 53, and the second spring 55 is disposed between the bush 56 and the bracket 52. In use, under low amplitude oscillations the bush 56 remains static relative to the rod 50 and only the spring 54 is effective without damping; however under large amplitude oscillations, the bush 56 reciprocates relative to the rod 50 to provide damping as both springs 54 and 55 deflect cyclically.

In each embodiment, the two springs may be of the same or different spring rates, and/or they may display non-linear characteristics. The first spring (i.e. spring 28 in Figure 4, and spring 54 in Figure 7) may be provided with damping, e.g. by providing piston 25 in Figure 4, or guide 53 in Figure 7, with friction. Preferably the degree of damping provided to the first spring (28 or 54) is relatively small compared with that provided to the second spring (31 or 55).

It will be appreciated that the suspension units which have been described may be used in the inverted position from that described, i.e. with the piston rod 24 (Figure 4) or rod 50 (Figure 7) fixed to the machine chassis and shaft 29 (Figure 4) or guide 53 (Figure 7) fixed to the tub and drum assembly.

It will be appreciated that the suspension units in accordance with this invention operate independently of the initial displacement of the tub and drum assembly relative to the chassis of the washing machine (and therefore independently of the load in the drum). Preferably the arrangement is such that the first spring (28 or 54) is not compressed to a solid condition at the point that the second spring starts to compress: accordingly a smooth transition occurs, without impact, between the undamped and damped stages of operation of the suspension units.

Claims

1) A suspension unit which comprises two opposed mounting elements, first and second spring elements operatively arranged between said mounting elements, and damping means for damping cyclical deflections of said second spring element, and arranged so that under small amplitude loading oscillations the first spring element deflects cyclically independently of said damping means and under larger amplitude loading oscillations cyclical deflections of the second spring element are damped by said damping means.

2) A suspension unit as claimed in claim 1, in which the first and second spring elements are disposed in series between the two mounting elements.

3) A suspension unit as claimed in claim 2, in which said damping means comprises a damping element disposed between the first and second spring elements and arranged for frictional sliding movement.

4) A suspension unit which comprises two opposed mounting elements, first and second spring elements operatively arranged between said mounting elements and a damping element disposed between said first and second spring elements and arranged for frictional displacement relative to one of said mounting elements.

5) A suspension unit as claimed in claim 3 or 4, comprising a tube in which the first and second spring elements are disposed, and said damping element comprising a piston frictionally displaceable relative to said tube.

6) A suspension unit as claimed in claim 5, in which one of said mounting elements comprises a piston engaged with said first spring element and displaceable relative to said tube.

7) A suspension unit as claimed in claim 3 or 4, comprising an elongate element carrying the first and second spring elements, said damping element being frictionally displaceably mounted on said elongate element.

8) A suspension unit as claimed in any preceding claim, arranged so that cyclical deflections of said first spring element are damped.

9) A suspension unit as claimed in claim 8, in which the cyclical deflections of said first spring element are damped to a smaller degree than cyclical deflections of said second spring element.

10) A suspension unit as claimed in any preceding claim, in which the first and second spring elements are of different spring rates.

11) A suspension unit as claimed in any one of claims 1 to 9, in which the first and second spring elements have substantially the same spring rate.

12) A suspension unit as claimed in any preceding claim, in which the first and second spring elements comprise different portions of a common spring or resilient body.

13) A suspension unit as claimed in any preceding claim, in which said first and second spring elements comprise coil springs.

14) A suspension unit substantially as herein described with reference to Figures 4 and 6 or Figure 7 of the accompanying drawings.

15) A washing machine comprising a tub and drum assembly, the suspension system of which comprises one or more suspension units as claimed in any preceding claim.