MSO1210 Mechanics Assessment

Patient Hoist Design 

You have been asked to evaluate the performance of a patient hoist (sometimes known as a patient  lift). There are many such items on the market and the one you are evaluating is based on this type  of design. More details are here. 

Overview 

The hoist is a ceiling mounted type and Figure 1 shows a drawing of the side and bottom of a section  of rail with the hoist mechanism (Housing assembly) attached. 

The Lifting Mechanism 

This is in a housing that contains a motor and a gearbox that drives a pulley. This is show in Figure  2. 

This is not complete as there would also be a rechargeable battery and a motor controller in the housing as well as some braking or ratchet mechanism for the pulley. The Pulley Assembly consists of a pulley and a length of webbing that winds up on the central spool. This is shown in Figure 3 where the spool diameter is 90 mm, and the outer diameter is 105 mm; the thickness of the webbing is 1 mm.

The rail comes in a number of lengths to suit the rooms in which the hoist is used. Top and bottom views are shown in Figure 4 that shows the position of the holes used for mounting to the ceiling.

For the purpose of calculation, this can be treated as a beam simply supported at the mounting holes  and a point load acting at the position of the centre of the housing (see Figure 1). The cross section  of the rail is shown in Figure 5, from which the second moment of area can be approximated by  ignoring the corner fillets dimensioned as R5 (TYP) and R3 (TYP) on the drawing. 

The Task
All of the drawings that are featured in this document are available of my learning to view. You will be asked to analyse the system given a random length of rail (L).
Maximum capacity (patient mass) of the motor / gearbox system in the housing is 160 kg to 220 kg.
You will be assigned a value in this range as well as other other random specifications to evaluate the performance characteristics. From these you will need to determine:
1. The minimum torque required to begin lifting a patient of maximum mass
2. The actual torque required to achieve full lifting speed within a given time
3. The speed of the motor at full lifting speed in rads/s and RPM
4. Using a 24 V motor with a given speed under load, the ratio required in the gearbox
5. Given a gearbox efficiency, the torque required by the motor and hence its power requirement and the current it will draw.
6. Given the section of rail in Figure 5, evaluate the second moment of area and the position of the neutral plane from the top and / or bottom of the rail.
7. Given a length of rail (L) evaluate the central deflection under full load as well as the maximum bending moment and maximum stress. Take Young’s modulus to be 210 GPa, g = 9.807 ms-1.
8. Given a yield stress of 350 MPa evaluate the factor of safety for the rail under full load.
It is advised that you set up a spreadsheet with the appropriate formulae so you can then put in the random specifications and establish the answers.