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Elevating Elderly Life.

Redesigned stairlift incorporates DC motor to increase lift capability and decrease mechanism size

A large percentage of the British population resides in narrow, multi-story dwellings with the bedrooms typically upstairs--an arrangement not usually conducive to elderly or disabled people. Government health insurance underwrites the cost of caring for this growing population and prefers the more economical installation of a stair lift rather than a move to new housing when one becomes disabled. This government funding, has in turn spurred technical innovation in the UK stairlift industry, leading to the adoption of DC motor drives as an improvement over older AC motor designs which still dominate the US market.

The Cumbria stairlift made by Lift Able Ltd, Stock-ton-on-Tees, UK, is used to convey elderly and disabled people who either sit on the seat with their feet on the footrest or, for those with knee-trouble, stand upright. When the start button is pressed continuously the chair travels on a rail up or down the staircase, slightly above the steps. This compact, straight-rail stairlift is intended for installation on straight staircases, although with modifications it can often be used to cover one or two steps to the left or right at the top of a main flight.

Overall Design

The lift consists of an upholstered swivel seat with two foldable armrests mounted to a motor unit. The seat can be swiveled at the top of the stairs to obtain a safer mounting and dismounting position for the user. The center of the swivel is offset to move the seat further onto the landing. When not in use, the seat and footrest can be folded away so as not to obstruct normal pedestrian use of the stairs. The seat is adjustable to suit the inclination of the stairs.

The motor unit rides on horizontal and vertical rollers in an extruded aluminum track with a rack and pinion drive system. The rollers ensure correct alignment as it travels up and down the track. Safety sensitive panels are fitted to stop the lift if it runs into an obstruction during travel, and an overspeed device stops the lift immediately if for any reason the normal running speed is exceeded. The footrest--also adjustable to the stair inclination--is fitted with safety sensitive panels to stop the lift if it runs into an obstruction during travel. A hand-winding device moves the unit manually in the event of an electrical failure.

The DC motor is powered by a 24 volt, 7A/hr battery which is kept on constant charge from house voltage through the lift controller. In some IBID versions, recharging power IBID supplied to the moving motor unit through a flexible cable, which is held in tension by a counterweight traveling inside the aluminum track. Later designs substitute contacts at the top and bottom of the stairs to recharge the battery, dispensing entirely with the traveling cable. A battery isolator is fitted at the rear of the motor unit. The constant-pressure push buttons for direction control are mounted on the armrest of the seat and on the wall at the top and bottom of the stairs. Stop switches are also fitted at these positions and a lockable keyswitch is mounted in the seat armrest. Optional infra-red remote controls are available for use instead of the wall mounted call/send controls. Easily adjusted stop limit blocks are fitted at the top and bottom of the track to adjust the normal stopping position. A final emergency limi t switch is fitted with cams at top and bottom of the track.

Drive Mechanism

The drive mechanism works through a self-locking wormgear drive with electric motor and fail-safe electromagnetic brake. The 1/3 hp, 250W motor is a type 33A5FEPM-GB, produced by Bodine Electric Company, Chicago, IL. The original lift design's AC motor was powered directly from the household electric supply and therefore ceased operation during power outages; the DC version, however, provides up to 20 return journeys while the main power is off. Additionally, the high-volt, external AC trailing cable was thought to be unsightly and tended to wear out with use. This cable made it difficult to design for curves in the staircase; the DC battery can be charged through contacts at the top and bottom of the staircase, thereby eliminating the cable's disadvantages.

Other reasons for change from AC to DC motor were based on noise considerations and the requirement for soft start and stop. Previously, the only control over acceleration/deceleration with the AC motor was provided by a large flywheel on the motor shaft lending additional inertia. The DC motor controller allows adjustment of the acceleration and deceleration rates in order to have smoother, safer operation for users. Typical starting torque for an AC motor is less than three times the rated torque, whereas the DC motor's starting torque is up to ten times the output torque of 42 N-m. A fail-safe electromagnetic brake prevents the seat and carriage unit from descending the slope of the track in the absence of electric power. Additionally, the DC motor weighs less than 2/3 of the AC motor and is almost 3.5 cm shorter and more than 2.5 cm narrower, making the mechanism more compact.

The gearbox can withstand forces applied under full load conditions. A drive pinion of pitch circle diameter 80.5 mm is mounted to the gearbox output shaft and drives through an idler pinion of equal size into the rack. The pinions have twenty-three teeth and are machined from forged steel to specification EN24T. The whole drive mechanism is mounted and enclosed within a steel frame to maintain alignment and give rigidity to the assembly.

Overspeed Device

The overspeed device is a positive drive unit (as opposed to friction drive) mounted within the main chassis frame. The drive is obtained from a chain sprocket machined integral with the idler pinion. The chain drives a centrifugally-operated activator, which in turn releases a toothed cam if the carriage speed exceeds the design speed. This toothed cam progressively engages with the rack and prevents any further downward movement of the carriage. The device can only be reset by a qualified person.

Seat Unit

The seat unit mounts to the motor unit with one large bolt and adjusts for inclination of the stairs by two leveling bolts with lock nuts. The seat plinth contains a large thrust bearing arranged with locking levers so that the seat can be swiveled through a maximum of 90 degrees and locked in position. An electrical interlock switch prevents the lift from being operated until the seat is returned to its running position and locked in place. Either one of the armrests can be fitted with the chair-mounted user controls. Both the seat and armrests can be folded when not in use. The seat's backrest is of a slim design so that the lift is suitable for narrow staircases.

Track

The track is a hollow extruded aluminum section anodized to give a nice appearance as well as a lasting and clean finish. Slots are extruded into the section to provide easily adjustable fixing positions for the mounting brackets. A channel is extruded to accommodate the rack which is retained in position by self-tapping screws as well as by a block at each end of the track. The retaining blocks also form mechanical stops at the extremes of travel so that the carriage cannot become disengaged from the track.

The track is normally fixed to the stairs with three pairs of brackets. One pair are mounted one step up from the bottom and a second pair mounted one step down from the top of the stairs. The remaining pair are fixed approximately halfway along the track. The brackets can be continuously moved in the slots to obtain the correct positions on any staircase. Two sizes of brackets are available to give the a wide range of mounting possibilities.

Overall, the DC stairlift has many benefits over the earlier AC version. It installs easily and plugs directly into a wall outlet--unlike the AC version which requires the services of a certified electrician to hard-wire the high-volt line. The DC motor is more compact than its AC predecessor, allowing the entire unit to be less obtrusive. Also, the DC motor is quieter and operates more smoothly with an inherently higher starting torque than an AC motor can provide. It is capable of carrying users weighing over 250 lbs on a stair angle range from 25 to 52 degrees at speeds up to 150 mm/sec, or 10 round trips per hour. The lift has been tested and approved by British Standards Testing Services and is CE marked. Having proven itself in use for two years in the UK, this model is currently being introduced to the US market, and is anticipated to receive ready acceptance due to the benefits of its DC motors.
COPYRIGHT 2001 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2001 Gale, Cengage Learning. All rights reserved.

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Author:Gooch, Stephanie
Publication:Designfax
Date:Aug 1, 2001
Words:1477
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