Block and Tackle

Our Design Rationale

Description and Rationale 

When initially presented with the design challenge, we considered two broad spaces: a rack and pinion machine as well as a block and tackle design. After initially prototyping and testing both designs, we opted for the block and tackle design due to its simplicity, high potential efficiency, and high iterability with total mechanical advantage (transmission ratio). Simplicity of design gave our team the ability to intensely focus on and refine a few sources of energy losses and mass inefficiency, resulting in a more refined final assembly. There are two main components of our machine: the block and tackle and the support stand. 

The block and tackle features two independent shafts, each consisting of 8 v-grooved pulleys which rotate on ball bearings. These independent shafts are inherent to any block and tackle design, closely emulating systems found on sailing ships as well as hauling systems used in mountain crevasse rescues. A major design decision was to only use a single set of pulleys, as opposed to a compound pulley system. With a single set of pulleys, there is one moving rope and two shafts. The addition of another set of pulleys would require two more shafts (increased mass), an additional connection to the support stand (increased mass and potential interference effects), another separate moving string, and an increased height of the support stand to accommodate the additional pulleys and string travel distance (increased mass). Ball bearing pulleys were used to increase the energy efficiency, and the size of the pulley well exceeds 4 times the string diameter: the smallest allowable for synthetic rope. 

The support stand is a simply supported 3-force member structure consisting of two 0.825” OD (0.0375” wall thickness) carbon fiber tubes that are loaded principally in compression. The mass was optimized for compression and buckling failure modes, and the in-plane nature of the loading forces allowed for a planar design. The acrylic feet were added to the bottom of the carbon fiber tubes to increase the stability of the support stand, as well as accounting for some error and variability in the planar nature of the forces. The design of the support stand was strongly influenced by an inability to fabricate and dramatically alter the carbon fiber (i.e. anything beyond cutting and drilling simple holes), due to the inherent health risks and lab safety restrictions. The steel shaft is directly inserted and epoxied to the carbon fiber tubes, adding rigidity to the support structure as well as countering any uneven torques experienced by the top shaft due to uneven tension in the cable as a result of friction and inefficiency losses. The position of our machine straddles the lever, allowing the reaction forces to balance the pulling force which prevents any tipping of the machine. 

Transmission Design

The major design decisions regarding the block and tackle transmission were overall ratio and mitigation of energy losses. As stated above, the single-stage block and tackle design was chosen for mass efficiency, lack of interference effects, and refineability through simplicity. Our overall transmission ratio is 16:1. The block and tackle mechanical advantage works inversely with the string distance traveled. If the weight can travel 2m for a desired lift of 0.1m, a 20:1 ratio would be the ideal ratio. However, effects such as string deformation, stretch, and subtle machine flexing preclude this ideal relationship. Additionally, the catalog lengths of shafts and the number of pulleys they could fit across restricted our ratio further down to 16:1. The bottom shaft could not interact with the support structure and the pulleys had minimum thicknesses, constraining the width of the machine. As shown below in the transmission analysis, adding more pulleys does increase energy loss, but not at an exponential rate, therefore justifying staying as close to the ideal ratio as the fabrication and design constraints would permit. We chose ball bearing pulleys with v-grooves to minimize energy losses within the transmission, used small washers between pulleys to decrease friction, and added lubrication to decrease losses within the bearings. We had purchased a braided fishing line to use for the transmission due to its mass efficiency and low friction and deformation, but decided to use the kevlar provided string after a testing failure where the string snapped.