Weights & Winding

The weights and winding subsystem is the power source and determines the winding period for the clock. There are many preexisting designs that could meet the functional requirements. We design a gravity powered system using large weights that is able to be rewound using a custom ratchet and pawl mechanism. Those two design components are brought together using a winding barrel, transmission shaft, and a key way shaft. The following sections will outline the design decisions we made, our manufacturing processes, and outcomes. 

Force on Pawl = 21.88 N

Once the weight needed was determined, we looked into material choices. The 13.12 lb requirements (6.56 lb each) and the space available for the weights were the main factors at play. The weights needed to drop at least 15.7in due to the winding barrel radius to reach a two hour winding period. The weights would be initially wound to 28in off the ground. This means the weights could have a max length of 12.3in each. To have extra time to drop, we wanted the weights to have a max height of 7in. Additionally the weights had a maximum cross-sectional area. There was a 4in x 2in drop area on each side of the clock. The goal cross-sectional area was 3in x 1.5in to allow for maximum clearance. Using these specifications we needed a material with a minimum density of 6350 kg/m^3. We decided to use stainless steel as stock was available to us for free and it had an density of 7860 kg/m^3.  The weights were then machined to sized, taped to attach to eye-bolts, and then tied to the cord. Paracord was the cord of choice as it was rated for holding up to 100lb which is a factor of safety over 7.

The gear on the winding shaft would be a 30 tooth gear with a 3 in pitch diameter. Using solid works drawings and hand sketches the following ratchet and pawl design was made. It pulls design features from commonly occurring ratchet + pawl mechanism. The gear would be printed with ABS to a thickness of 1/4in which was determined during gear train design. Using that standard through our project for material thickness I had to determine if ABS would be strong enough for the pawls. 

To move forward into the manufacturing process it was necessary to show that the pawls would perform under high torques. I needed to know that the pawl would 1) not deform, 2) not fracture, 3) and have a factor of safety above 2. Finite element analysis was conducted under the conditions calculated by the weights. The results are below. With only one pawl taking on the torques, it will not fracture, deformation will be minimal, and it has a minimum factor of safety of 4.5. The deformation needed to be within 0.1 mm and the FEA assumes a deformation of only 0.0162 mm. The FEA supports the ratchet+pawl design. For additional security two pawls will be engaged at all times. 

In addition to the weights, winding barrel, and ratchet/pawl mechanism other components were bought or designed and manufactured. These parts did not undergo in depth analysis . As engineers we determined that they were not likely to fail and follow good mechanical design practices. The main shaft is a steel key-wayed shaft. Grooves were machined to hold retaining rings which secured components from moving along the shaft. The key in the key-way secured the components from rotating independently of the shaft. The shaft is supported by one ball bearing on either end. Shaft collars separate the winding barrel from the ratchet as retaining rings could not be placed. On the end a winding key was designed. This winding key is 3d printed out of ABS. It uses a key with the key way to turn the ratchet wheel to rewind the weights after they fall.  This full assembly is pictured below. 

The weights and winding subsystem is the power source of the clock. It is not in charge of regulating the distribution of the power only providing it when engaged. Through understanding its iterations with other subsystems we were able to determine the functional requirements. Component were designed with appropriate analysis and literature based understanding. Finite element analysis was conducted under the conditions calculated by the weights. The results are below. EA models and mathematical models validated design choices prior to manufacturing. Once manufactured, all of the components functioned correctly. The weight estimated was not enough to power the clock and a large torque is needed. This makes sense as we did not know the inefficiencies of the other subsystems and machined the weights ourselves. When additional weight is applied the the weights unwind powering the gear train and regulated by the escapement.