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Vibration Testing Machine
University of VictoriaFaculty of Engineering
Summer 2010 Work Term ReportVibration Testing Machine:Design, Construction and AnalysisRocky Mountain BicyclesDelta, British ColumbiaJames HerriotV00657090
Work Term 3Mechanical Engineering
firstname.lastname@example.orgAugust 31, 2010
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Table of Contents31. Summary:
43.1 Principal of operation:
63.2 Design considerations:
83.3 Construction and Machining:
List of Figures
5Figure 1 Junkers Vibration testing machine
5Figure 2 Transverse shear
6Figure 3 Proposed Machine layout
7Figure 4 Graph
7Figure 5 Custom anodized aluminum hardware
7Figure 6 Bolt holder
8Figure 7 Offset shaft and sleeve
9Figure 8 Finalized design
10Figure 9 Bottom plate with needle bearings
10Figure 10 Bolt inserts
11Figure 11 Torque specification
12Figure 13 Assembled Dry 1:08
12Figure 12 Wear plate
13Figure 14 Assembled with grease ~25 sec
14Figure 15 Completed machine
This report covers the design and construction of a custom transverse vibration testing machine. The machine was built for Rocky Mountain Bicycles so that their R&D department could test various fasteners used in off-road mountain bikes. Severe vibrations during trail rides can cause the pivot screws to loosen which poses a major safety issue for riders. In order to solve this problem, different fasteners were considered. Different types of thread locking compounds, mechanical screw locking devices, and other designs were developed to solve this problem. A machine which could consistently and predictably test the self loosening characteristics of each option would be needed. The machine was modeled after the Junkers Testing Machine that was developed in 60s. The custom machine has an eccentric main shaft driven by an electric motor. The eccentric shaft is attached to the top plate and forces the top plate to vibrate on needle bearings. The offset shaft has an offset center which produces a set vibration amplitude. Since making the offset shaft would be time consuming, it would be cost prohibitive to make multiple shafts with different offsets. A double eccentric shaft was made to overcome this issue. This double eccentric shaft would make it possible to adjust the amplitude of vibration. The machine was successfully constructed and produced useable results.2. Introduction:Rocky Mountain Bicycles was established in 1981 and was one of the first companies to make true off-road mountain bikes. Rocky Mountain continues to push the boundaries of materials and technology and riders have come to expect nothing but the best from their rides. This is why Rocky Mountain focuses so much effort on research and development, all of which takes place in their R&D Facility located in Delta, BC. During a demo ride of a new full suspension bike one of the test riders noticed several of the pivot screws loosened. This is a common problem on full suspension mountain bikes as they are subject to constant and severe vibration loads when ridden off road. This can pose a serious hazard to the rider and led to discussions in the R&D room about different types of thread locking compounds, mechanical screw locking devices and other designs which could eliminate this problem. It was decided that a method of testing these different thread locking compounds and locking devices was necessary for comparison and analysis. A machine which could consistently and predictably test the self loosening characteristics of different types of fasteners would be needed. Web research was conducted which lead to the Junkers transverse shear vibration testing machine. This machine is used by laboratories to test bolted connections for their self loosen characteristics (see Fig 1). The machine works by producing a small amplitude transverse shear movement to the bolted connection. This small shearing action (Fig 2) is enough to loosen an untreated bolt in less than a minute. The machine would have to be heavily modified to suit the needs of R&D. Therefore, a custom Junkers testing machine that could accommodate bicycle parts needed to be designed and constructed.
3. Discussion: 3.1 Principal of operation: The testing machine was modeled after a Junkers transverse vibration testing machine. The Junker's transverse vibration-loosening test provides a simplified method for broad scale testing and inspection of the transverse vibration loosening properties of fasteners. The test machine is able to generate relative motion in the clamped parts perpendicular to the axis of the fasteners. The Junkers method provides quantitative results relating the variables of clamp-load, number of cycles, and amplitude. From Nordic Steels website: A testing machine to compare the relative self loosening characteristics of different bolted connections was developed in 1966 by Gerhard H. Junker. With this machine it is possible to analyze the locking characteristics of fasteners under transverse loading conditions. In this test a bolted shear connection is moved by an eccentric rotating engine. Due to an elastic centerpiece the deformation controlled load is transformed into a mixture of deformation and force. The Junker test is standardized by the German regulation DIN 65151. With the Junker test it is not possible to affirm a secure connection but to compare different connections and safety devices.
The machine has an eccentric main shaft which is driven by an electric motor. A pair of bearings fit onto the eccentric shaft (Fig3), which transmit the vibrating motion to the top plate. The top plate slides on needle bearings and supports the clamped load of the bolted connection. The top plate vibrates under the head of the fully tightened bolt (Fig 2), requiring the machine to develop considerable force to do so.
As the machine vibrates, the bolt gradually comes undone, losing its clamping effectiveness. This can be seen visually by watching the head of the screw come loose, or by using a force sensor. The force sensor can measure the clamping tension of the bolt and using analysis software, a graph showing the connection coming loose over a period of time can be made. By comparing the time it takes to loosen a connection, it is possible to gauge the loosening resistance of the fastener.
3.2 Design considerations: After several discussions with the other members of R&D a set of design criteria was produced. The machine would have to be able to test different sizes and lengths of screws and bolts. This is because the hardware used to put together the bikes frames is usually different for each model, see Fig 5. This proved to be a challenge as the testing machine commonly used is only designed for one specific length of bolt. To accommodate the different sizes and lengths an insert would have to be fitted into the machine somehow. It would have to be easy to replace, easy to duplicate if other sizes were necessary, and fit precisely so that it would not vibrate with the bolt.
It was decided that a large cylindrical insert, held in place by the clamping force of the bolt would be ideal. This would be easy to make on the shop lathe, using available steel round stock (Fig 6). Another issue that came up was choosing the amount of eccentricity for the main offset shaft. The offset shaft has an offset center which produces a set vibration amplitude. It was questioned what the offset should be, and whether one offset shaft would be enough to test a variety of bolts. Since making the offset shaft would be time consuming, it would be cost prohibitive to make many of them with different offsets. The idea of making a double eccentric offset shaft was designed to overcome this problem. This consists of two parts, the eccentric main shaft and the eccentric sleeve. The vibration amplitude can be adjusted by rotating the sleeve on the shaft, see Fig 7. The eccentric shaft has an amplitude rage of 0 to 3mm. There is an extra sleeve which has a bigger offset, giving a range of 1.5-5mm.
It was also decided that the machine should be able to vibrate an entire bike frame to test the bolts in place and to see if they loosen in the frame. This necessitated a larger mounting table with a jig to hold the bike frame in place while it was vibrated. The design was finalized; the designed machine could perform all the required tasks and could fit a force sensor for later use.
3.3 Construction and Machining:
Construction began with sourcing material and parts. The machine shop has a good selection of aluminum and steel material, and the machine was designed to use as much material from the shop wherever possible to save cost and procurement time. The 1.5hp motor, pillow blocks, and v-belt sheaves were recovered from Rockys assembly facility in Saint-Georges, Quebec. The motor cord had been cut in s