Intro: Disabled Hoist Lift
A family member suffering from Motor Neurone Disease (MND or ALS) and needing transport between two floors in a small house. The impairment is equivalent to tetraplegia (complete loss of muscular function, inability to sit upright and need for specialised hoisting) and results in the requirement to transport the individual either directly in the wheelchair or in a hoist sling.
The commercial solution on the UK market is a through-floor lift carrying the wheelchair, which causes unacceptable loss of space (a room on each floor) and interventions to the property structure. In other countries a 'ceiling lift' is sold, but cost is expected to be very high (several tens of thousand Euro).
A DIY lift construction inspired by the chairlifts of skiing resorts and made mostly out of wood.
Step 1: The Concept
The stairs to be negotiated have one 90-degree bend at the top landing and a sufficiently spacious lower landing. The ceiling height exceeds 2.3m (7ft 6in). This lends itself to constructing a rectilinear suspension track to transport the individual in the hoisting sling between two wheelchairs, one positioned at the lower landing and the other wheeled onto a simple platform at the top.
The support structure consists of five gantries attached to the floors and the stairwell (Figure 1). A winch provides both hoisting from and to the chairs and translation along the track (Figure 2 left-hand side). The critical components permitting to realise this solution with just off-the-shelf parts and wood are the trucks and wheels of a skateboard. A carriage, very much like an upside-down skateboard, runs on an L-shaped track suspended from the gantries (Figure 2 right-hand side).
The design philosophy hinged on three principles:
- Components choice and dimensioning were based on the mechanical properties demonstrated in other applications, eg skateboard trucks are designed to withstand shock forces far exceeding our requirements.
- A simple design with a minimum of components kept the system transparent and the manufacturing effort low. The design method was heuristic with many details being worked out during manufacturing.
- The validation approach was empirical, meaning that parts and assemblies were tested during construction with twice the expected service load (equivalent to 125 kg).
Note that a fourth critical principle is being applied: Since the device has been in service it is closely monitored and regularly inspected.
Thus the lift could be built on a 450GBP materials budget and in short time using only DIY hand tools (hand saw, jig saw, hacksaw, drill, drill press, files, wood chisel, hammer, screw driver, level and measuring tape).
Step 2: Gantries and Track
The gantries were built out of softwood (fir) beams of 38 mm X 63 mm (1.5 in X 2.5 in) cross-section and 2.4 m in length. For the transversal beams, sections with as little knots and irregularities as possible were chosen. The beams rest on the gantry posts (Figures 3 and 4), they are held in place by steel L-brackets attached with screws in pre-drilled holes and are free to bend. No other holes weaken the beams. The posts are attached to the floor and stairwell with brackets and screws (Figures 4 to 6). If no anchoring point was available, the post was stabilised with a 45 degrees support (buttress - Figure 7). If the length of the posts was insufficient, an extension was attached with linear brackets on either side (Figure 8).
The track was built out of 18mm-thick hardwood plywood board (density > 400kg/m^3), 3.5mmX30mm wood screws and wood glue. The track's profile is L-shaped and obtained by attaching the 40mm wide inner strip to the 50mm wide outer strip of plywood board with glue (applied under compression) and screws at 100mm intervals (Figure 9). The screws orientation is alternating (from inside then outside) and 3mm diameter screw holes were pre-drilled. The inner and outer board strips were staggered to provide an overlap area 150mm long between different sections of the construction.
The track is suspended to the gantry beams through partly open, C-shaped box constructions (Figures 9 to 12), which are free to slide along the beam. The suspension was obtained by directly shaping the outer board of the track to include a hanging profile (Figures 9, 11 and 12). The suspension for the middle (third) gantry differed slightly for no other reason than the practicality of cutting the board straight. An opening in the shape of the gantry beam but 6 mm larger (44 mm X 69 mm) was drilled out. The minimum width of the hanging profile around an opening was 30 mm. All corners were rounded to reduce stress concentrations. Where the track changes inclination (from flat to 50 degrees) the profile was rounded with a radius of 100 mm (Figure 9).
The track can be adjusted to accommodate the available space and other constraints (eg different wheelchairs - Figure 13). Also and quite importantly, it is worth noting that the design proved quite forgiving with respect to manufacture and measurement tolerances and (slight) errors.
The track was mounted in three sections, all previously assembled with glue and screws. These were slid on the gantry beams and then all was lifted into place and attached to the gantry posts - I somehow managed to do this on my own, so two people can easily achieve this. The different sections of the track are then joined together at the overlapping sections, which are attached using screws 50 mm apart and no glue.
Finally, the longest section of track between two gantries is 1650 mm long.
Step 3: Carriage, Winch and Platform
The carriage was made out of the same board used for the track. Two solid skateboard trucks and square-profile wheels determine the separation between the two sides of the track. With only 4 mm clearance on each side, a wheel rests on 14 mm of track. The wheel rubber helps to spread the load of the four wheels onto the track. The trucks are fastened to the board with high-tensile steel machine bolts and self-locking nuts (Figure 14). Two aluminium strips were used as washers for the 8 bolts and as reinforcement for the board (Figure 15). Four rollers on the sides of the board, two at the front and two at the back of the carriage, maintain the carriage straight within the track within a couple of millimetres of tolerance. Two looped anchor plates (from a commercial swing system - Figures 14 and 15) provide hooking points for the sling and winch cable. To ensure safe retention these are passed through slits in the board rather than being screwed onto it. An E-shaped component was fashioned out of the same plywood board to house vertical axles for two skateboard bearings (one either side) and the cutouts are designed to provide clearance whilst moving along the track. This was attached to the board with wood screws and reinforced with wooden buttresses (Figures 15 and 16). This, remember, keeps the track from opening up. The forces involved during normal operation are quite small, but because it is quite a critical component I intentionally over-dimensioned it making it no narrower than 30 mm at any one point. A retention carbine hook is attached to the rear truck with braided sailing line. A transport carbine hook is similarly attached to the bottom of the carriage. The purpose of these hooks and various pieces of line is shown in the next section.
A standard commercial hoist winch (the sort used on building sites - Figures 17 and 18) was attached to the topmost gantry on a dedicated beam as high as possible. The beam is oriented with its larger cross-sectional dimension horizontal and is anchored to the gantry posts with brackets front and back. The winch is positioned with the cable spool centred in the track and mounted with steel bolts and self-locking nuts. The winch is allowed to rotate on its support beam to accommodate the wide range of cable angles necessary for the operation. Unfortunately, the automatic mechanical stop ring of the winch had to be removed because it impeded correct operation - a custom made one could be fashioned possibly out of wood (but I just left that). The pulley provided with the winch is attached to the closest carriage ring (Figures 15 and 17).
The top platform is simply a rectangle of the trusted plywood board, hinged on a transversal beam and having a side step to stop the wheelchair from rolling off and a front lip to roll on (Figures 19 and 20).
The bill of materials for this project is attached.
Step 4: Operation
The images in this section illustrate the operation of the lift. My lovely assistant Liz aptly dressed in home attire helped modelling this section.
- The wheelchair is wheeled beneath the track facing the wall (Figure 21) - this allows the later removal of the chair.
- The sling is hooked to a spreader board, which has two loops of line of different lengths (Figure 22) - the spreader is a common part of hoist systems and keeps the sling from closing onto the individual.
- The shorter line loop is attached to the winch hook. The retention carbine hook is attached to a line from the lowest gantry beam (Figure 15) - this retains the carriage during the subsequent hoisting.
- The individual is hoisted with the winch (Figures 23 and 24). The wheelchair is removed.
- The transport carbine hook is attached to the longer loop (Figure 25). The winch hook is lowered (Figure 26) and detached (Figure 27). The retention carbine hook is detached.
- The individual is transported up the stairs (Figure 28).
- The retention carbine hook is attached to a line from the fourth gantry beam (Figure 29). The carriage is winched until this line is in tension. A safety hook is also attached (Figure 17) - this is to avoid any unwanted sliding down.
- The winch hook is lowered, attached to the shorter loop and the individual is hoisted up. The transport carbine hook is released (Figure 30).
- The chair is wheeled in position and the individual is hoisted down into it (Figure 31).
Transport in the opposite direction follows the inverse sequence of steps.
In conclusion, it should be noted that the structure is quite flexible and will bend during operation, a behaviour expected with the materials used and not an issue. The ride is not quite smooth on account of the abrupt function of the winch, but this has been no problem with my relative.