Analysis of Design and Results

5.1 Kinematic Analysis

Assumptions:
i) The wheel assembly rotates at approximately 12RPM, in order to provide sufficient torque to carry the walker up and down the stairs at a reasonable speed.

ii) Since on average a human walks at around 1-3mph, when the walker is being used as motorized walker each of the 6inch wheels should rotate within the range of 120-200 RPM in order to operate at an appropriate speed.

iii) The diameter of the motor pinion gear is 1.25 inches.

iv) The diameter of the driver gear of the shaft is 3inches.

Calculations:

1) Gear ratio of the motor pinion gear and the driven gear on the main shaft is given by the following:

Since the planetary gear motor operates at approximately 50RPM, in order to reduce this to 12RPM on the main shaft to provide the required torque (previously calculated in the static analysis to be 675inlb) the size of the driven shaft gear is given as:

Where the rpm of the pinion gear=50RPM

the rpm of the driven gear on the main shaft=12RPM

, the diameter of driven gear

, the diameter of the pinion gear =1.25inch

Now,

Therefore gear ratio of pinion to driven gear is 1:4.

2) Compound gear ratio

Since the average human walks within a speed range of 1-3mph, therefore the drive wheels will be made to operate within this range. In order to achieve this, the following computations were made:

Where, , is the tangential velocity of the drive wheels =1-3mph=0.45-1.34m/s

R, is the radius of the drive wheels= 3inches

, is the angular velocity of the drive wheels

Now, the angular velocity will be calculated by using the upper value of the speed range (1.34m/s).

Since is known the rpm of the drive wheels is given as:

Therefore the gear ratio of the compound gear set with respect to the input

and output RPM is given as: 1:……. (1:14)

Where, , is the rpm of the driver gear on the shaft.