Universal Screwdriver

Transcription

1 Universal Screwdriver Design Team Sean de Laforcade, Katey Farel Matt Lapinski, Josh Peterson Design Advisor Prof. Andrew Gouldstone Abstract Current screwdrivers use mating surfaces between the driver and screw head to apply torque. For installing and removing screws. While this method is successful for screws and drivers with matching geometries, users require multiple screwdrivers to access the wide range of screw sizes and shapes used in consumer goods. The objective of this project was to explore the design space of materials and geometries that would allow low torque application to a range of household screw sizes and shapes with a single tip. An explored preliminary design space included granular packed cells, magnetism to increase normal force and friction between driver and screw head, and malleable heads to increase surface-to-surface contact between driver and screw head. Experimental testing and design iteration led to the final product, a malleable head reinforced with a metal plate.the malleable element of this design will allow the driver to mate with the recessed geometry of the screw, while the reinforcement plate will give the driver strength to apply the required torque for inserting and removing screws. The composite geometry allows mating with a range of screw heads, and sufficient torsional stiffness to insert or remove that range of screws. For more information, please contact a.gouldstone@neu.edu.

2 The Need for Project No single screwdriver tip is capable of providing torque to a wide variety of screw sizes geometries. Such a screwdriver, if created, could replace an entire set of drivers. The screwdriver is one of the most common tools in the world. Despite its useage, no screwdriver currently exists that has the ability to apply torque to a wide range of screw sizes and geometries. Users need to buy multiple screwdrivers or a kit that has removable tips for each screw. A single screwdriver that could work on a large variety of common screw types and sizes will hold a competitive advantage over the universal screwdriver kits currently on the market. The team is creating a product that will appeal to hobbyists and people who like DIY projects. The Design Project Objectives and Requirements Design an inexpensive universal screwdriver that can apply torque to a wide variety of household screws with a single tip. Design Objectives The screwdriver must be able to effectively drive common household screw types and sizes. Common household screws include hex, Torx, Philips, flat, and Pozidriv and range from machine screw sizes 0 to ¼. The screwdriver must apply torque easily and consistently, without damaging itself and/or the screw not requiring excessive applied pressure. The product may be sold on the market in two forms. The first is a full screwdriver including the handle, shank, and tip. The second is a hex style tip for use with universal screwdriver and ratchet kits currently on the market. This will allow customers to buy either a complete universal screwdriver, or supplement an existing universal screwdriver kit. Design Requirements The screwdriver must apply at least 30 in-lbs of torque, which is comparable with the torque an average human can apply with other hand held screwdrivers on the market. During torque application, the rotational position of the handle, shaft, and head must be maintained. This means that the head, shaft and handle must rotate evenly under torque.

3 Design Concepts Considered The team explored multiple The screwdriver should not damage the screw, so torque may methods of torque only be applied through the interaction of the exterior surface of the transmission from screwdriver screw and the screwdriver tip. The team decided that this interaction to screw. The preliminary could be a result of mating surfaces (like conventional screwdrivers), concepts were based on magnetism, enhanced friction, or a combination of the three. surface to surface interaction Conventional screwdrivers rely solely on mating surfaces to between the driver and screw. transmit torque. Since a truly universal screwdriver must be able to drive screws with different recessed geometries, its tip must be malleable in order to conform to different screws. Pressure-dependent strength and stiffness would allow the tip to be malleable, but with the application of normal force, exhibit the ability to effectively transmit torque. Solid rubber and tightly packed granular material exhibit pressure-dependent properties suitable for preliminary testing. Polyurethane rubber was chosen for the solid geometries due to its high tear strength, high elastic modulus, and low cost. Stock polyurethane rods of durometers 40A, 60A, 80A, and 90A were purchased and cut to shape using an ultrasonic cutter. The most successful durometer was determined through experimental analysis. 80A was found to easily conform and the effectively apply torque. Tightly packed granular material was tested by filling rubber membranes with coffee grounds. The packed cells held the shape of the screw geometry after insertion, and the area of the packed cell that was inside the geometry of the screw took the shape well. However, the rubber membranes tore easily, and the concept was abandoned due to its inability to withstand cyclic loading. The second preliminary concept, magnetism, was eliminated for a few reasons. Firstly, magnetism itself would only be applicable to ferrous screws. Any non-ferrous screw would require another form of surface to surface interaction to drive. Secondly, the space inside the shank is limited, making it difficult to create an inductor that produces a strong enough field to hold a ferromagnetic screw enough to drive it. Finally the cost and difficultly of assembly lead the team to discard this concept.

4 Enhanced friction is still a concept being considered, as it is easy and inexpensive to embed both polyurethane and metal with a particles of aluminum oxide. This would provide extra grip between screwdriver head and screw geometry without cosmetically damaging the screw. Recommended Design Concept The recommended design The results of the preliminary experimental analysis showed concept is a thin stainless steel that transmission of torque from driver to screw is more successful with reinforcement plate with a stiff and strong material reinforcing one that conforms to the recessed polyurethane cast on front and geometry of the screw. Experimental analysis also proved that torque back faces. is most effectively transmitted via sharp corners, due to locally high stiffness and strength of nonlinear elastic materials under loading. Design Description Since conforming fully to the recessed geometry is no longer the main goal of the design, a rigid plate may be used to reinforce malleable material. To design this plate, dimensions of each screw size and geometry were evaluated. Common geometry was identified by superimposing Phillips, Torx, hex, and flat geometries, the four most common geometries (see Figure 1). End points of the greatest common diameter can be referred to as torque points. The plate is dimensioned with the width of each step determined by the distance between torque Figure 1 points of each screw size, and the vertical distance between these steps determined from the ANSI standard for depth of each screw size. The thickness of this plate was chosen to fit the smallest screw size within the scope. Two side plates were also designed to prevent the main plate from deforming during torque application. 304 stainless steel was chosen for its stiffness, strength, ease laser cutting, and ease of laser welding for prototyping and production. An opening through the plates Figure 2 allows polyurethane rubber to be cast as a single piece around the plate (see Figure 2). The geometry of the polyurethane rubber replicates the most successful solid rubber tips from preliminary testing, maximizing contact with as many screw head geometries as possible. The polyurethane rubber does provide any additional torque points until screw size 4 so that it will not interfere with small slotted screws. All screw sizes above 4 will be contacted by the polyurethane rubber to allow for more torque points (see Figure 3). Figure 3

5 Analytical Investigations Computer simulated FEA shows an increase of stiffness and strength on corners of solid polyurethane rubber under loading. Under loading conditions expected from a hand held screwdriver, localized stiffness at these corners increased by about 2.5 times. Experimental Investigations Experimental analysis of recommended concept was successful. The tip was able to effectively transmit torque to hex, Philips, and slotted screws from sizes Design iteration is required for use on ¼ screws. Key Advantages of Recommended Concept Screwdriver will be price competitive in current market. Although not proven yet, side reinforcement plates improve fatigue resistance allowing for long life. This product allows users to have a single screwdriver for common screw sizes and types with a single tip. Financial Issues Good for mass production. Estimated COGS under $8. Tooling will be relatively expensive. Production costs will initially include tooling required for the casting and turning of the handle, welding of the reinforcement plates to stock, and mold for casting polyurethane rubber. However, piece prices will be very low, especially at production volumes. The hex bit version of the product will be less expensive to manufacture than the full screwdriver. Recommended Improvements Design could improve by optimizing polyurethane rubber geometry as well as improving the reinforcement plate with varying thickness. To improve this design, the reinforcement plate could taper to be thicker as the width increases. A better polyurethane rubber design/mold could increase the number of torque points with many screws. During the project, the team was limited by the resolution of the fused deposition modeling 3D printing. Ideally the reinforcement plate would be thicker in the sections designated for the larger screws and thinner in the areas for smaller screws. This could be achieved in the future by casting the reinforcement plate.