EP0175782A1

EP0175782A1 - Improvements in chairs and beds for the disabled

Info

Publication number: EP0175782A1
Application number: EP85902069A
Authority: EP
Inventors: John Francis Barratt
Original assignee: Wincanton Engineering Ltd
Current assignee: Wincanton Engineering Ltd
Priority date: 1984-04-07
Filing date: 1985-04-04
Publication date: 1986-04-02
Also published as: GB2168249A; GB8409072D0; GB8529823D0; GB2168249B; WO1985004570A1

Abstract

Chaise ou lit pour handicapés, possédant un châssis (10, 70, 85) possédant deux organes latéraux, et des courroies flexibles (11-13, 71, 88) s'étendant autour des organes latéraux et formant une surface du support pour un patient. Les extrémités d'une des courroies au moins (11-13, 71, 88) sont reliées par des câbles (47, 49) au levier basculant (45) d'un dispositif de tension (15, 72, 90) monté de manière détachable sur le châssis (10, 70, 85) au-dessous de la surface de support, le levier basculant (45) pouvant pivoter sous l'action du dispositif (15, 72, 90) pour serrer et relâcher périodiquement chaque courroie (11-13, 71, 85), de manière à supporter le corps du patient à différents endroits l'un après l'autre et réduire ainsi le risque de formation de plaies de décubitus. Dans un lit chaque courroie (71) est relâchée tour à tour tandis que les autres courroies sont maintenues tendues. Une variante de chaise possède une couverture de siège (86) pourvue d'une ouverture (87) réduisant la pression sur les tubérosités ischiatiques du patient, une courroie flexible (88) s'étendant à travers l'ouverture (87) sur le côté inférieur du couvercle du siège (86), la courroie (88) étant munie d'un rembourrage (89) en matériau souple monté sur l'ouverture (87), et un mécanisme de tension (90) servant à serrer et à relâcher la courroie (88) périodiquement.Chair or bed for the disabled, having a frame (10, 70, 85) having two lateral members, and flexible straps (11-13, 71, 88) extending around the lateral members and forming a support surface for a patient . The ends of at least one of the belts (11-13, 71, 88) are connected by cables (47, 49) to the rocking lever (45) of a tensioning device (15, 72, 90) mounted so detachable on the chassis (10, 70, 85) below the support surface, the rocking lever (45) being able to pivot under the action of the device (15, 72, 90) for periodically tightening and loosening each belt (11 -13, 71, 85), so as to support the patient's body in different places one after the other and thus reduce the risk of pressure sores. In a bed each belt (71) is released in turn while the other belts are kept tensioned. A chair variant has a seat cover (86) with an opening (87) reducing the pressure on the patient's ischial tuberosity, a flexible strap (88) extending through the opening (87) on the side bottom of the seat cover (86), the strap (88) being provided with a padding (89) of flexible material mounted on the opening (87), and a tension mechanism (90) used to tighten and release the belt (88) periodically.

Description

Improvements in Chairs and Beds for the Disabled.

This invention relates to chairs and beds suitable for use by a patient who is paralysed or disabled and confined to a chair or bed, and the invention is concerned more particurlarly with chairs and beds which are adapted to prevent or reduce the risk of pressure sores in the body of the patient.

It is known from British Patent Specification 1009787 to provide a bed whose supporting surface consists of transverse flexible straps connected at their ends to power-driven means mounted at the sides of a fixed frame and operable to gradually tighten and then slacken alternate straps so that the body of a patient on the bed is supported in different places in succession. The power driven means consist of longitudinal beams or cranks arranged one on each side of a fixed frame and hydraulic or pneumatic motors for moving the beams or cranks. The object of the invention is to provide an improved construction of a chair or bed adapted to support the body of a patient in different places in succession.

According to the present invention there is provided apparatus forming a chair or bed for a patient, comprising a frame and means including at least one flexible belt mounted on said frame and providing a supporting surface for the patient, wherein the frame has two side members on opposite sides of the frame, the flexible belt extends across the frame and around the side members, and the ends of the belt are connected to power-driven tensioning mechanism mounted on the frame below the supporting surface and operable periodically to tighten and then slacken the belt, the belt when tightened supporting part of the weight of a patient on the supporting surface, and the belt when slackened supporting little or none of the weight of the patient. The power-operated tensioning mechanism can be controlled by an electronic time sequencer of any suitable construction known in the art.

In the apparatus of the invention, the frame can be of any suitable standard construction which has two side members suitable for mounting the flexible belt thereon. The power driven tensioning mechanism preferably comprises a modular unit which is readily detachable from the frame and from the ends of the associated belt.

In one construction of apparatus according to the invention and forming a chair, the supporting surface comprises three flexible belts extending transversely across the frame and around side members of the frame, the belts being arranged one behind the other in the fore-and-aft direction of the chair, and two power-operated tensioning mechanisms are mounted on the frame below the supporting surface and operable periodically to tighten and slacken the central belt and the rear belt respectively. In another construction of apparatus according to the invention and forming a chair, the means providing the supporting surface comprises a seat cover mounted on the frame, the seat cover having an aperture for relieving pressure on the ischial tuberosities of a patient sitting in the chair, and said flexible belt extends across the aperture on the underside of the seat cover, the part of the belt below the aperture being fitted with a pad of resilient material arranged so that the pad is raised into the aperture in the seat cover and into a position in which the top of the pad is above the seat cover when the belt is tightened by the tensioning mechanism, and the pad is lowered within the aperture when the belt is slackened by said mechanism.

In the apparatus of the invention forming a chair, the frame may of course be of any suitable construction and may be provided with feet for resting on a floor or may be provided with wheels as in an invalid chair.

In apparatus according to the invention and forming a bed for a patient, the means providing the .supporting surface may comprise a plurality of flexible belts which extend across the frame and around the said side members, the belts being arranged one behind the other in the fore-and-aft direction of the bed, and a plurality of power-operated tensioning mechanisms are mounted on the frame below the supporting surface, each tensioning mechanism being operable periodically to tighten and slacken a separate one of said belts.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: -

Figure 1 is a side elevation of the seat portion of a chair having a supporting surface formed by flexible belts in accordance with the invention for support of a patient, the lower ends of the legs and the back of the chair being shown cut away for convenience,

Figure 2 is a cross sectional elevation of the seat portion of the chair of Figure 1 taken along the line II -II in Figure 1 ,

Figure 3 is a detail view of a clamp which is detachably mounted on the main frame of the chair and which supports a sub-frame fitted with mechanism for tensioning the flexible belts, Figure 4 is a side elevation of a bed having a supporting surface formed by flexible belts in accordance with the invention for support of a patient,

Figure 5 is a plan view of the bed of Figure 4, Figure 6 is a perspective view of the seat portion of a chair having a supporting surface formed by a seat cover and a flexible belt in accordance with the invention for support of a patient, Figure 7 is a side elevation of the seat portion of the chair shown in Figure 6, and

Figure 8 is a cross sectional elevation of the seat portion of the chair of Figure 7 taken along the line VIII-VIII in Figure 7. The chair of Figure 1 comprises a main frame 10, three flexible belts 11, 12, 13 extending transversely across the frame 10 and arranged one behind the other in the fore-and-aft direction of the chair to form the supporting surface for a patient seated in the chair, and two power-operated tensioning mechanisms 15 supported on a sub-frame 16 detachably mounted on the main frame 10, the two tensioning mechanisms 15 being operable periodically to tighten and slacken the central belt 12 and the rear belt 13 respectively.

The main frame 10 is formed of circular section tubular steel bar and comprises two upright front legs 20 interconnected by a cross bar 21, two upright rear legs 22, two upper side members 23 arranged in a horizontal position one on each side of the main frame and each interconnecting the front and rear legs of the associated side of the main frame, and two lower side members 24 arranged in a horizontal position one on each side of the main frame and each interconnecting the front and rear legs on the associated side of the main frame.

The three belts 11, 12, 13 are each formed of a washable non-stretchable flexible material which is permeable to air to enable excess moisture on the skin or clothes of a patient to evaporate through the belts. The belts are preferably formed of a fine net woven fabric. The three belts 11, 12, 13 are mounted on and extend between the two upper side members 23 of the main frame 10. The front belt 11 has its ends extending around the two side members 23 and secured together by any suitable means so as to hold the front belt taut in the form of an endless band. The ends of the central belts 12 and the rear belt 13 extend downwards at the sides of the chair. The fabric at each end of the belts 12, 13 is turned over and seamed to form a transverse pocket 25 for reception of a bar 26, and the centre portion of the pocket is cut away at 27 as shown in Figure 1 to expose the centre portion of the bar 26.

The sub-frame 16 comprises a front plate 30 (Figure 1), a rear plate 31 , two side plates 32, 33 (Figure 2) welded to the underside of the plates 30, 31 four tie rods 34, 35. 36, 37 extending between the front and rear plates 30, 31 and secured thereto, and a deep tray 39 the walls of which are formed with flanges 40 welded to the undersides of the side plates 32, 33. The four tie rods are all arranged in the fore-and-aft direction of the chair, the tie rods 34, 35 being arranged one at each side of the sub-frame adjacent the upper edges of the plates 30, 31. the third tie rod 3β being positioned between the tie rods 34, 35, and the fourth tie rod 37 being positioned below the tie rod 35. Each of the tie rods has reduced diameter screw-threaded ends which extend through apertures in the plates 30, 31 , the tie rods being secured to the plates by nuts 41 screwed on the ends of the tie rods. The sub-frame 16 is detachably secured to the lower side members 24 of the main frame by four clamps 42 arranged on each side member 24. As shown in Figure 3, each clamp 42 comprises upper and lower jaws 43 secured together by a bolt 44 which extends through apertures in the two jaws 43 and through an aperture in the associated side plate 32, 33, the head of the bolt engaging against the underside of the associated side plate 32, 33. The clamps are shown in Figure 1 but are omitted from Figure 2 to avoid obscuring other details of the construction.

Each of the tensioning mechanisms 15 comprises a rocking lever 45 pivotally mounted intermediate its ends on the tie rod 36, the lever 45 being approximately in an upright position, a linear actuator 46 operable to pivot the lever 45, a cable 47 extending around a pulley 48 on the tie rod 35 and having one end secured to the adjacent end of the associated belt 12 or 13 and the other end secured to the upper end of the rocking lever, and a further cable 49 extending around a pulley 50 on the tie rod 34 and having one end secured to the adjacent end of the associated belt 12 or 13 and the other end secured to a part of the rocking lever below the tie rod 36. The rocking lever 45 comprises two spaced apart plates 51 secured together by pins 52, 53 at the upper and lower ends respectively of the plates 51. A screw-threaded rod 54 extends through an aperture in the pin 52, one end of the rod being formed with an eye 55 and the other end of the rod being fitted with a wing nut 56. The cable 47 is formed at its ends with loops 57, 58, the loop 57 being engaged around the bar 26 at the cut-away portion 27 in the adjacent end of the associated belt, and the loop 58 being engaged with the eye 55 on the screw-threaded rod 54. The cable 49 is also formed at its ends with loops 59, 60, the loop 59 being engaged around the bar 26 at the cut-away portion 27 in the adjacent end of the associated belt, and the loop 60 being engaged around a pin 61 extending between the plates 51 of the rocking lever 45. The pin 61 is fitted to the part of the rocking lever below the tie rod 36, and the distance between the axes of the pin 61 and the tie rod 36 is the same as the distance between the axes of the pin 52 and the tie rod 36.

The linear actuator 46 consists of a body 65 formed with a cylinder 66 , a plunger 67 axially movable in the cylinder, an electric motor 68 mounted on the body, and gearing (not shown) drivably connecting the motor to the plunger and operable to impart axial movement to the plunger in response to rotational movement of the motor. The body of the actuator is formed with a transverse aperture through which the fourth tie rod 37 extends as a rotational fit, amd the head of the plunger 67 projects between the lower ends of the two plates 51 of the rocking lever 45 and is formed with an aperture which receives as a rotational fit the pin 53 at the lower end of the rocking lever. The linear actuator is thus supported on the tie rod 37 and the pin 53 on the rocking lever. The tray 39 supports an electric battery and an electronic time sequencer (not shown) for controlling supply of electric current from the battery to the motors of the two actuators at selected time intervals. When the plunger of the actuator is extended the rocking lever 45 is pivoted in a clockwise direction about the tie rod 36 as viewed in Figure 2, so as to slacken its associated belt which then hangs downwards between the two side members 23 of the main frame. When the plunger of the actuator is retracted the rocking lever 45 is pivoted about the tie rod 36 in an anticlockwise direction as viewed in Figure 2, so as to draw its associated belt into a taut condition with the part of the belt between the two side members 23 substantially level with the front belt 11.

The tensioning mechanism for each of the belts 12, 13 is adjusted by setting the linear actuator 46 with the plunger 67 retracted fully into its cylinder and then screwing the wing nut 56 along the rod 54 so as to draw the associated cables 47, 49 and the belt into a taut condition. The proportion of the weight of a patient supported by each of the three belts 11, 12, 13 may be regulated by adjustment of the wing nuts 56 of the two mechanisms 15 so as to adjust the tension in the central and rear belts 12, 13.

When the chair is being used by a patient who has to remain seated in the chair for a long period of time, the tension in the central and rear belts 12, 13 is automatically adjusted at selected time intervals, by operation of the linear actuators, so as to slacken each of the belts 12, 13 in turn while maintaining the tension in the other of the two belts 12, 13 and thereby reduce the risk of pressure sores in the body of the patient. As an example, the electronic time sequencer may be set so that in every ten minute period the actuator for the rear belt is operated to slacken the rear belt for a period of two minutes, thereby removing pressure entirely from the patient's seat in the area overlying the rear belt. The rear belt would then return to the taut condition and the other linear actuator operated to slacken the central belt 12 for a two minute period. There is not normally any need to reduce the tension of the front belt 11, providing the feet of the patient are at a suitable height on foot rests on the chair, since the area behind the knees is not prone to pressure sores, and also the knees can be lifted by the patient or by an attendant.

The two belts 12 and 13 can readily be removed from the main frame of the chair for cleaning by operating the actuators so as to slacken the belts and then removing the bars 26 from the pockets 25 in the ends of the belts so as to disconnect the 15 belts from the cables 47, 49. The sub-frame 16 together with the two tensioning mechanisms 15 and the cables 47, 49 can then readily be removed from the main frame of the chair as a unit for servicing or replacement by disconnecting the clamps 42 from the lower side members 24 of the main frame. The chair of Figures 1 and 2 may be used to support a patient when providing toilet facilities. For this purpose, with the patient on the chair, a padded and shaped bar is slipped onto the chair at the back and both the belts 12, 13 are slackened and removed. Toilet facilities can then be supplied without lifting the patient out of the chair, the patient being supported on the front belt 11 and the back bar. When the toilet facilities are no longer required, the back bar is removed and the two belts 12, 13 replaced.

The bed shown in Figures 4 and 5 comprises a main frame 70 having a plurality of flexible belts 71 extending transversely across the frame 70 and arranged one behind the other in the fore-and-aft direction of the bed to form the supporting surface for a patient on the bed, each belt being connected to a separate power-operated tensioning mechanism 72 (only shown in broken lines in Figure 5) mounted on the frame of the bed and operable periodically to tighten and slacken the associated belt. The power-operated tensioning mechanisms can conveniently be of similar construction as that shown in Figures 1 and 2 and have therefore not been shown in Figure 4.

The main frame 70 of the bed has two upper side members 74 and two lower side members 75 reinforced at intervals along the bed by vertical struts 76. The belts 71 are mounted on and extend between the two upper side members 74, and the ends of the belts extend downwards at the sides of the bed. The belts 71 can conveniently be of greater width than the belts on the chair of Figures 1 and 2. Each end of the belt is turned over and seamed to form a pocket for reception of a metal bar, as in the belts of the chair of Figures 1 and 2, but the pocket is cut away at two positions to expose the bar, and two short cables 78 connect the exposed bar to a plate 79 which is in turn connected by a strap 80 to the rocking lever of the associated tensioning mechanism. The straps 80 extend around the underside of the lower side members 75 of the frame. The tensioning mechanisms of the bed of Figures 4 and 5 are periodically operated under the control of a computer programmed so that each belt 71 is slackened in turn while all the other belts are maintained taut, in order to avoid pressure sore developing in the body of a patient on the bed. The computer is also preferably arranged so that a nurse, by operation of a suitable control, can programme the computer to slacken any selected number of belts periodically while holding the remainer of the belts taut to relieve pressure of the patient's body on the supporting surface whatever the length of the patient. The power operated tensioning mechanism for each belt is mounted on a sub-frame which is detachably mounted on the main frame of the bed below the belt by clamps similar to those shown in Figure 3, so that the sub-frame and the tensioning mechanism can readily be removed for servicing or replacement. The computer on the bed is preferably arranged so that a nurse can slacken any two adjacent belts merely by operating an appropriate control button on the computer. The two belts can then be removed to provide access to the underside of a patient. The nurse can thus carry out toilet facilities on any part of the patient without the need to lift the patient. When the toilet facilities are completed, the nurse replaces the belts, operates the computer to tighten the belts, and recommences the sequence of slackening each belt in turn. A rigid board (not shown) is preferably mounted on the frame of the bed below the supporting surface for the patient, the board extending the full length of the of bed and covering the tensioning mechanisms. The board can then conveniently be used as a worktop during the carrying out of toilet facilities. In the event of cardiac arrest in a patient on the bed, the periodic slackening of the belts could reduce the effectiveness or interefere with the appropriate remedial action. For this reason, the computer preferably has a control which, when operated, immediately tightens any belts which are slack and maintains all the belts taut.

The chair of Figures 6, 7 and 8 comprises a main frame 85, a flexible seat cover 86 extending transversely across the frame, the side edges of the seat cover being anchored to the frame and the rear portion of the seat cover having a large aperture 87, a flexible belt 88 extending transversely across the frame and below the aperture 87 in the seat cover, the belt having a pad 89 of soft resilient material arranged to project upwards through the aperture 87 when the belt is taut,and a power-operated tensioning mechanism 90 supported on a sub-frame 91 detachably mounted on the main frame by clamps 42 similar to those shown in Figure 3, the tensioning mechanism being operable periodically to tighten and slacken the belt 88 so as to raise and lower the pad 89 within the aperture. The seat cover 86 and the pad co-operate to form the supporting surface for a patient seated in the chair. The main frame 85 of the chair is similar to the frame of the chair of Figures 1 and 2 and like parts are denoted by like reference numerals. However, the frame 85 has in addition four brackets 94 (Figure 8) welded two to each of the upper side members 23 of the frame, the brackets on each side member 23 being inclined downwards and inwards towards the brackets on the other side member 23, and two auxiliary side members 95 each extending between the two brackets 94 on a separate one of the two upper side members 23, each member 95 being parallel to its adjacent side member 23. The main frame 85 is also provided with two side bars 97 (Figure 7) arranged one on each side of the chair and each bolted at its ends to the front and rear legs 20, 22, and six toggle fasteners 98 secured three on each of the side bars 97 at spaced intervals along the side bars. The seat cover 86 is formed of a washable non-stretchable flexible material which is permeable to air and is preferably a fine net woven fabric. The seat cover 86 is mounted on the upper side members 23 of the frame with its side edges extending downwards at each side of the frame, the seat cover 86 extending substantially the full length along the side members 23. The fabric at each side of the seat cover is turned over and seamed to form a pocket 99 for reception of a bar 100, and the pocket is cut away at three positions 101 to expose the bar 100. The seat edges of the seat cover 86 are anchored to the main frame by 25 the toggle fasteners 98. Each toggle fastener comprises a channel-section casing 105 arranged in an upright position with the base thereof bolted to the associated side bar 97, a channel-section lever 106 having its side walls pivotally connected at 107 to the side walls of the casing 105 adjacent the 30 upper ends thereof, the lower end of the lever 106 being fitted with a handle 108, and a rod 109 having one end attached to the handle and the other end formed with a hook 110, the rod projecting upwards with its hook 110 engaged with the exposed bar 100 at one of the cut-away positions 101 on the seat cover. The levers 106 of the toggle fasteners are pivotable between a locked position shown in Figure 8 in which the seat cover 86 is held taut, and an unlocked position in which the seat cover is slackened. In the locked position, the lever 106 is nested within 5 the casing 105 and extends vertically downwards with the rod 109 passing on the outer side of the pivot 107, that is between the pivot 107 and the base of the casing. The lever is pivotable inwards about the pivot 107 into an unlocked position in which the rod 109 is on the inner side of the pivot 107 and displaced 10 upwards. The flexible belt 88 is mounted on and extends between the two members 95, and the ends of the belt extend downwards at the sides of the chair and are connected to two cables 47, 49 in the same way as the belts 12, 13 of the chair of Figures 1 and 2. The parts of the belt 88 between the pad 89 and each of the members 95 are fitted with heavy weights 112 so that the centre portion of the belt 88 carrying the pad 89 is substantially level. The tensioning mechanism 90 and the sub-frame 91 are of the same construction as the tensioning mechanism 15 and the sub-frame 16 of the chair of Figures 1 and 2, except that only one tensioning mechanism 90 is provided, and like parts are denoted by like reference numerals in the Figures . The tensioning mechanism 90 is arranged so that, when the plunger 67 of the acutator 46 is in the fully retracted position, the belt 88 is in a taut condition with the upper part of the pad 89 projecing above the part of the seat cover extending between the side members 23, as shown in full lines in Figure 8. When the plunger 67 is in the fully extended position, the belt 88 is in a position in which the top of the pad 89 is below the part of the seat cover extending between the side members 23, as shown in broken lines in Figure 8. The seat cover 86 is arranged so that, when a patient is sitting in the chair, the ischial tuberosities of the patient are positioned over the aperture 89. The seat cover 86 and the sub-frame supporting the tensioning mechanism can be adjusted as necessary in the fore-and-aft direction to align the aperture with the ischial tuberosities of a particular patient by adjustment of the positions of the toggle fasteners 98 along the side bar 97 and of the position of the clamps 42 along the lower side members 24. The seat cover 86 is held in the fore-and-aft position by front tapes (not shown) which are attached to the two front corners of the seat cover and hold the front of the seat cover wrapped around a front cross bar 113 on the frame of the chair as shown in Figure 6, the front tapes then extending around the front legs rearwardly along the sides of the chair, and then around the back of the chair where their ends are joined together by an adjustable fastener. The rear of the seat cover is provided with four tapes secured by adjustable fasteners to a cross member on the back of the chair. In use, the buttocks of the patient, which act as cushions to protect the ischial tuberosities, are in contact with the pad 89 when the plunger of the actuator is retracted, the buttocks compressing the pad down to a level at which the top of the pad is substantially flush with the part of the seat cover extending between the side member 23. The pad 89 and belt 88 then support part of the weight of the patients When however the plunger of the actuator is extended to lower the pad, the ischial tuberosities are relieved of pressure. Operation of the tensioning mechanism is automatically controlled as desired by an electronic time sequencer.

It is to be understood that the chairs and bed illustrated in the drawings are only examples of embodiments of the invention, and their components may be altered or arranged as desired. For example, the flexible belt or belts may be mounted on pulleys on the frame of the apparatus to reduce 30 friction forces. Each flexible belt may be in the form of an endless band and the tensioning mechanism operable to pull down the lower run of the band. The tensioning mechanism may be bolted direct to a base plate forming part of the frame of a chair or bed.

Claims

CLAIMS:

1. Apparatus forming a chair or bed for a patient, comprising a frame and means including at least one flexible belt mounted on said frame and providing a supporting surface for the patient, wherein the frame has two side members on opposite sides of the frame, the flexible belt extends across the frame and around the side members, and the ends of the belt are connected to power-driven tensioning mechanism mounted on the frame below the supporting surface and operable periodically to tighten and then slacken the belt, the belt when tightened supporting part of the weight of a patient on the supporting surface, and the belt when slackened supporting little or none of the weight of the patient.

2. Apparatus as claimed in claim 1 , wherein the means providing the supporting surface comprises a plurality of flexible belts which extend across the frame and around said side members, and an adjacent two of said belts have their ends connected to power-driven tensioning mechanism operable periodically to slacken each of said belts in turn while holding the other belt taut.

3. Apparatus as claimed in claim 2 and forming a chair for a patient, wherein the means providing the supporting surface comprises three flexible belts extending transversely across the frame and around said side members, the belts being arranged one behind the other in the fore-and-aft direction of the chair, and two power-operated tensioning mechanisms are mounted on the frame below the supporting surface and operable periodically to tighten and slacken the central belt and the rear belt.

4. Apparatus as claimed in claim 1 and forming a bed for a patient, wherein the means providing the supporting surface comprises a plurality of flexible belts which extend across the frame and around said side members, the belts being arranged one behind the other in the fore-and-aft direction of the bed, and a plurality of power-operated tensioning mechanisms are mounted on the frame below the supporting surface, each tensioning mechanism being operable periodically to tighten and slacken a separate one of said belts.

5. Apparatus as claimed in claim 1 and forming a chair for patient, wherein the means providing the supporting surface comprises a seat cover mounted on the frame, the seat cover having an aperture for relieving pressure on the ischial tuberosities of a patient sitting in the chair, and said flexible belt extends across the aperture on the underside of the seat cover, the part of the belt below the aperture being fitted with a pad of soft resilient material arranged so that the pad is raised into the aperture in the seat cover and into a position in which the top of the pad is above the seat cover when the belt is tightened by the tensioning mechanism, and the pad is lowered within the aperture when the belt is slackened by said mechanism.

6. Apparatus as claimed in any of the preceding claims, wherein the power-driven tensioning mechanism comprises a modular unit which is readily detachable from the frame and from the ends of the associated belt.

7. Apparatus as claimed in claim 6 in which the frame comprises two upper side members and two lower side members , the or each belt being mounted on and extending between the two upper side members, and the power-driven tensioning mechanism being mounted on a sub-frame detachably mounted on the lower side members.

8. Apparatus as claimed in claim 7, wherein the tensioning mechanism comprises a rocking lever mounted intermediate its ends to the sub-frame for rocking movement about a pivot axis, a linear actuator mounted on the sub-frame and having a plunger pivotally connected to the rocking lever whereby axial movement of the plunger imparts rocking movement to the lever, and two cables connected to the ends of the associated belts and to the rocking lever on opposite sides of the pivot axis thereof.

9. Apparatus as claimed in claim 8, wherein the linear actuator is pivotally mounted on the sub-frame about an axis parallel to the pivot axis of the rocking lever.

10. Apparatus as claimed in claim 9, wherein the sub-frame comprises a front plate, a rear plate and a plurality of parallel tie rods extending between the two plates, the two cables being mounted on pulleys rotatably mounted on two of said tie rods, the rocking lever being pivotally mounted on a third of said tie rods, and the actuator being pivotally mounted on a fourth of said tie rods.