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Mainframe, Shaft and Fixtures


The mainframe can be seen as the skeleton of the whole machine, it serves mainly 2 purposes: to support the components, and to adjust belt tension when the bearings are attached to it.


Design Features

The mainframe consists of a square metal sheet, 4 vertical pole, 6 horizontal poles and 8 L-shaped bars (L- bars). The horizontal poles and L-bars are attached to the vertical poles by screws and nuts. 2 major concerns when designing the poles is that 1) with a part size up to 1 meter, it is hard to ensure 100% accuracy and 2) the surface on which the poles will be placed is not entirely flat. Therefore as a design precaution, the holes on the poles have a relatively large clearance, as seen in the diagram; so that minor adjustments can be made when the poles are assembled together. When all the positions are set, the L-bars will be welded to a metal sheet which is fixed to the ground, enforcing the whole structure.

In the second level of the mainframe, 2 rows of pits are engraved in the horizontal bars, this is the mechanism for adjusting the tightness of the belts. Bearings are attached to the pits and are allowed to slide freely until the belt is tight, which is then fixed by screws and nuts. The procedures are shown in the diagram below:


The shafts are to serve one but very important purpose: to transmit the rotational energy from the motor or handle to the furnace.


Design Features

In order to allow the parts to have a better grip on the shaft, steel keys are inserted between the shaft and the parts. Initially there are 2 proposed ideas:1) the whole shaft will have a key on it. 2) Pits will only be made on the part of the shaft where a component is located. Finally the second idea is chosen since it saves a lot more manufacturing time and raw material. The keys will be made longer than needed in order to allow adjustments in case miscalculation occur.

According to metric standards for engineering (BS 4235 and BS4500), the tolerance of keyways for the rectangular parallel keys is –0.015 to +0 mm while the tolerance of holes for the shaft is 0.021mm.



The furnace is used to melt metals of up to 1800, it is therefore extremely important to prevent the furnace from revolving too much- 110°. Revolving beyond 110°might damage the furnace, as well as causing a threat to the user. Therefore a fixture was designed as a safety measure for the machine. During the manual operation, the fixture will restrict the user from revolving beyond 110°.


Initial Design

Upon designing, there are 3 proposed design on the fixture, as shown below:

  1. Ratchet: A mechanism consisting of a pawl that engages the sloping teeth of a wheel or bar, permitting motion in one direction only, and it is difficult to set the stopping point at exactly 110°








  1. Locking by Cap: A mechanism consisting of a circular disc and a cap attached to its surface, as shown in the diagram. A metal sheet restricts the revolving limit. Unlike ratchet, this design allows a 2-direction movement, however, the cap will have to sustain a large shear force, which may shorten its service life.








  1. Locking by Bar: A modification from the above design, as a solution to the shear force experienced by the cap, it is now replaced by 2 bars.  






Further Design

It was decided that the locking by bar mechanism would be the final design of the fixture. Upon further discussion within our group, we came to a conclusion that the fixture will have to be linked with the circuit that drives the motor. If the motor continues to operate when the fixture reaches the limit, it will cause damage to both the fixture and the motor. As a solution, a sensor is placed on each of the fixture bar, so that the motor will stop once the limit is reached.









Design Features

The fixture consists of 3 components, 2 L-bars and a circular disc with bars attached to its surface. The disc is to be placed on the shaft, while the L-bars are screwed on to the horizontal bar on the mainframe. Holes are drilled on the L-bars so that the sensors can be placed there in the future.