Designing a Test Rig - Coursework Example

Test rig Lecturer Introduction A test rig is essentially a set of apparatus that used for the assessment of electrical or mechanical equipments. Before equipment leaves the manufacturer’s premises to the final consumer, the manufacturer must ensure that the equipment operates as desired. This may be with respect to braking efficiency, as could be the case with the braking system of an automotive, securing attachment for wheelchairs and testing bearings capacity and efficiency among several other applications. Several test rigs have been designed and fabricated for commercial purposes. In some cases, test rigs are designed depending on the equipment to be tested. One example of a commercial test rig is the Crash Test Rig designed and fabricated by JK Controls Limited. The apparatus comprises of a trolley that hauls the sample being tested. The trolley, which moves along a track by rotating rubber cords, is stopped abruptly by applying a decelerating force using a solid beam (JK Controls Limited, n.d). The test rig facilitates the determination of the required braking force of equipment, which is dependent on the weight of the equipment, braking materials and the speed. Another example of a commercial test rig is the Aircraft Wing Brake Test Rig designed and fabricated by JK Controls Limited. The rig tests the braking system of an aircraft by subjecting a rotating shaft (of a prototype) to a brake that is electrically operated. The shaft, which rotates at 1500 rpm, has to be halted by the brake within 40ms (JK Controls Limited n.d). Therefore, the basic operating principle of the aforementioned test rig examples is the application of a braking force to a rotating element, either a wheel or a shaft. Rig Operation The test rig being designed will operate on the same basic principle, the application of a braking force to a rotating disk to prevent it from rotating. At equilibrium state, the braking beam will be in contact with the surface of the disk (stationary). The contact action will be achieved through a locking mechanism. Once the operator applies a force on the braking beam, at the contact patch, the braking beam retracts from the disk, which allows the disk to rotate. The test rig will be used for determining braking force required to stop a rotating disk within a given time. Therefore, the applied force will be used to disengage the braking system to allow the disk to rotate. Once the operator withdraws this force, the braking system will be engaged. Figure 1: Wheel and Braking Beam Rig Specifications 1. The disk will be rotated by a load that will be applied through its central axis. This load will produce a torque of 2.5NM. 2. The disk will be 100 mm in diameter and 20 mm thickness 3. The braking beam will be 100 mm in length, 20 mm in width and 5 mm in thickness 4. The operator will apply force on one end of the braking beam (20 mm length of one end of the braking beam) 5. The wheel and the braking beam are made of steel whose: a. Young’s Modulus is 200 GPa b. Yield Strength is 250 MPa c. Coefficient of static friction µs is 0.4 d. Density is about 7850 kg/m3 (Nelson, Best and McLean 1992: 328) Design considerations 1. Load application on the braking beam may only be applied on the contact patch as shown in figure 1. This load may be applied using a clamp, mechanical fixing or by use of an adhesive. 2. The wheel may be modified, for example, by providing for location holes and keyways for securing the when to the rotating shaft. 3. The wheel can be fitted to a shaft that has a diameter of 20 mm, which is the internal diameter of the wheel as shown in figure 2. 4. The test rig must be manual and mechanical. No electric components and equipment should be used. Figure 2: Wheel Figure 3: Braking beam Design requirements Based on the rig specifications and design consideration stated above, the test rig will be designed based on the following requirements. 1. Determination of whether the braking beam specified will sustain the load conditions 2. Determination of whether the shaft diameter will sustain the load conditions 3. Determination of whether the entire system will fail at its current state 4. Selection of the mechanical components, mechanisms and fastenings that will be used in the test rig to achieve the desired results through concept generation Concept Generation The basic operation principle of the test rig is similar to the Crash Test Rig and the Aircraft Wing Brake Test Rig presented above. However, the rig will contain several other components that will work together to achieve the ultimate goal, braking action. Concept generation is considered a brainstorming operation through which a designer establishes different methods of solving a problem (McKitrick 2011: 20). Based on the project specification, design consideration, cost consideration and material availability, the designer comes up with the best design concept to pursue to solve the problem under consideration (McKitrick 2011: 20). Other factors include machining requirements, availability of machining requirements, available technical knowhow, size and weight of the final system (portability), ease of use of the system (considering the end user) and maintenance/operating requirements of the system among others. When generating concepts, a designer can decide to evaluate existing systems and mechanisms or come up with entirely new mechanisms for evaluation. For the design under consideration, the design concept will be selected through an evaluation process of available mechanisms, which are not necessarily on available test rig designs. The concepts being evaluated are discussed in the section that follows. The final design concept is based on the following considerations: Ease of manufacture and assembly Ease of operation Number and complexity of components Size of the assembled system Weight of the assembled system Operating Principle Concept 1: Trolley wheel locking mechanism This locking mechanism is often found on shopping carts, tables (which have wheels for moving around) and vegetable racks among others. These locks are often easy to operate because an operator only needs to engage and disengage the lock. They are often held secure in the locking or unlocking position by a clipping mechanism, where they are clipped at the desired position. Figure 4: Trolley wheel locking mechanism The test rig will operate in a similar principle to the trolley wheel lock mechanism is as far as the braking principle is concerned. In other words, the braking beam will press hard on the wheel just like the lock presses hard on the trolley wheel preventing it from rotating. In order to provide for the on/off operation of the braking system, a spring or a clip may be used or operator may simply press the braking beam against the wheel. Concept 2: Aircraft Wing Brake Test Rig of JK Controls Limited The rig tests the braking system of an aircraft by subjecting a rotating shaft (of a prototype) to a brake that is electrically operated (JK Controls Limited n.d). A similar principle can be used in the test rig under consideration. However, based on the requirement that the system should be manual mechanical, the electrically operated brake should be replaced by a manually operated brake. Figure 5: Concept 2 Operation of the braking beam Concept 3: operator presses the braking beam hard against the wheel Two arrangements can be used on this concept. The first arrangement involves a pivot. The operator exerts a force on one end of the braking beam, which causes the opposite end of the beam to move in the opposite direction and get into contact with the wheel. A second arrangement, although extremely dangerous but inexpensive, is where the operator simply presses the braking beam on the wheel. Considering that the test rig shall be used for force determination, the two arrangements under this concept cannot be used because it will be impossible for the operator to evaluate the braking force required for a given wheel rpm. Figure 6: Concept 3 Concept 4: Spring This involves the use of a spring to press the braking beam against the wheel or retract the braking beam away from the wheel. In the first arrangement, a compression spring would be used to exert a force on the braking beam so that it gets in contact with the wheel. To allow the wheel to rotate, a force would be applied by the operator, which, through a pivot, will act against the spring, compress it and remove the braking beam from being in contact with the wheel. This arrangement will allow the operator to determine the force required to halt the wheel at a given rpm. According to Hooke’s law, a spring exerts a force Fs= -Kx, where k is the spring constant, and x is the spring displacement (Serway and Jewett 2011: 173). Therefore, it is possible to determine the force applied if the spring constant and the displacement (the length at which a spring is compressed from the equilibrium/free position). The second arrangement would make use of a tension spring that maintains contact between the braking beam and wheel, via a pivot, at equilibrium state (figure). To disengage the brake and allow the wheel to rotate, an operator applies a force on the contact patch, against the spring action, which forces the opposite side of the braking beam to move away from the wheel. Braking is activated when the operator withdraws the applied force. Like the previous arrangement, it is possible to determine the force required to halt the wheel (Fs= -Kx). Figure 7: Concept 4 Securing the wheel Concept 5: threads This would involve using a threaded shaft on which the wheel, which has internal threads, will be fixed. A nut can then be used to lock the wheel and avoid it from unthreading from the shaft. This approach will ensure a securely held wheel although it would be costly since it would require threading at least half of the shaft and the inner surface of the wheel. Further, the threads on the shaft may be damaged if not protected through covering, which would render the entire shaft useless. Concept 6: key and key ways Keys and keyways are often used for attaching pulleys, gears and other components on shafts to enable a shaft to transmit torque to the attached component without relative movement between the two (shaft and pulley, for example) (Gates 2009). It would be easy and inexpensive to manufacture since it would only require machining keyways on the shaft and bore. Further, it would provide for easy assembly while providing for a tightly fixed wheel. Design concept The final design concept incorporates a number of the aforementioned concepts with slight modifications. The principle of operation of the test rig is similar to the trolley wheel mechanism shown in concept 1. A compression spring, shown in concept 4, will be used for applying the braking force. The wheel shall be secured to the shaft using a key and a keyway as shown in concept 6. System Design Mass of the wheel: , where density = 7.85g/cm3 Mass (wheel) = 11837 g Shaft Design: The wheel shall be mounted on the frame using two pillow bearings, one on each side of the wheel. The width of each bearing shall be considered 50 mm and an allowance of 20 mm shall be provided between any two components (bearing 1 to the wheel and wheel to bearing 2). Therefore, shaft length is 160 mm and its diameter is 20 mm. This shaft is being subjected to two moments: twisting moment due to the braking action and bending moment due to the weight of the wheel. Figure 8: Shaft The torque transmitted by the shaft, T= 2.5 Nm The mass of the wheel was found to be 11837 g. Therefore, the weight of the wheel, W = 116.12 N A free body diagram of the setup is shown in figure (Figure number). Figure 9: FBD of the shaft The wheel is mounted midway between the two supports (bearings). Therefore, the maximum bending moment at the centre of the wheel, The Equivalent twisting moment, = 4.05 Nm Equivalent twisting moment (Te), Where t is the Yield strength = 250 mPa = 4.4 mm A shaft of diameter 4.4 mm would be sufficient. Therefore, a shaft of diameter 20 mm, based on the bore of the wheel will not fail. Considering a shaft diameter of 20 mm, = 97 Key and Key Way: Figure 10: Key and Keyway Figure 7 shows a key and a keyway with their respective dimensions, which depend on the shaft size as shown in appendix 1. Accordingly, since the shaft has a diameter of 20 mm, the keyway shall have a width (W) of 6 mm and a depth (h) of 2.8 mm, which shall be machined into the shaft. The key shall have a width of 6 mm and a depth of 6 mm, which implies that a 6 x 3.2 mm groove shall be machined on the wheel’s bore. The key and keyway shall have a length of 4 mm. Braking beam and spring force Tangential force acting on the wheel, = 50 N Force required to stop the wheel, Therefore, the spring must produce a force of at least 125 N. Considering that the operator can produce up to about 800 N of force with his leg, the braking beam shall be designed for a force ranging between 125 N and 800 N. The pivot shall be located 30 mm from the spring side as shown in figure (Figure number). The beam must resist bending and shear breaking. Figure 11: FBD of the braking beam Since yield strength of the material is 250 Mpa, the braking beam will not fail. Discussion Steel will be used in manufacturing the rig. Steel is readily available and it is inexpensive. Further, it is easy to work on, such as boring, machining (keyways) and turning. Steel is not brittle, and it is tensile to a given value, which means that it can accommodate a considerable amount of twist or bend before breaking. This makes it suitable for the shaft. However, steel wears out quickly through friction. Therefore, when used to make the wheel and braking beam, extensive wear will be experienced especially if high speeds will be used. This will demand frequent replacement of the wheel and braking beam in order to ensure consistent results. Compression steels are good at exerting a force, which is often used in cars. It shall be possible to determine the amount of braking force being applied. Further, it shall be possible to alter this force for different applications by adjusting the spring distance or changing the spring (using a spring of different spring constant value). However, springs often heat up if compressed and uncompressed too often. In such a case, the spring constant changes, which implies that the spring exerts a lower force than before, which is likely to compromise on the accuracy of the test rig. The final system will be easy to use because the operator only withdraws his foot from the braking beam to allow the spring to stop the wheel via the breaking beam. The force required to stop the wheel in a given time shall be determined based on the spring being used. Therefore, a single operator can operate the rig since after engaging the braking beam (by withdrawing his foot), the operator can immediately start a stop watch to determine the time taken by the wheel to stop completely. Bibliography Gates 2009, Product Application Notes: Shaft and Hub Keyway and Key Sizes, Accessed 21 June 2012 from http://www.gates.com/downloads/Vol%2056%20No%2003%20- %20Shaft%20And%20Hub%20Keyway%20And%20Key%20Sizes%5B1%5D.pdf JK Controls Limited. n.d. Test Rigs - Case Studies. Accessed 20 June, 2012 from http://www.jkcontrols.co.uk/test-rigs-cs1.htm McKitrick, MA 2011, The Complete Guide to Inexpensive Ideaing, Author, p. 20. Nelson, EW, Best, CL & McLean, WG 1998, Schaum’s Outline of Theory and Problems of Engineering Mechanics: Statics and Dynamics, Fifth Edition, McGraw-Hill Companies, Inc., p. 328. Serway, RA & Jewett, JW 2011, Physics for Scientists and Engineers, Volume 1, Mason, USA: Cengage Learning, p. 173. Appendix Appendix 1: Metric Standard Parallel Keyway and Key Sizes Table 1: Metric Standard Parallel Keyway and Key Sizes (Gates 2009) Metric Standard Parallel Keyway and Key Sizes Shaft Diameter (mm) Keyway (mm) Key (mm) From To Width (W) Depth (h) Width (W) Depth (T) 6 8 2 1 2 2 9 10 3 1.4 3 3 11 12 4 1.8 4 4 13 17 5 2.3 5 5 18 22 6 2.8 6 6 23 30 8 3.3 8 7 31 38 10 3.3 10 8 39 44 12 3.3 12 8 45 50 14 3.8 14 9 51 58 16 4.3 16 10 59 65 18 4.4 18 11 66 75 20 4.9 20 12 76 86 22 5.4 22 14 86 96 25 5.4 25 14 96 110 28 6.4 28 16 111 130 32 7.4 32 18 131 150 36 8.4 36 20 151 170 40 9.4 40 22 171 200 45 10.4 45 25 201 230 50 11.4 50 28 231 260 56 12.4 56 32 261 290 63 12.4 63 32 291 330 70 14.4 70 36 331 380 80 15.4 80 40 381 440 90 17.4 90 45 441 500 100 19.5 100 50 Appendix 2: Detailed Drawings Read More