DESIGN DEVELOPMENT OF AN INDIGENOUS TRICYCLE RICKSHAW
Phase 6: Planning for consumption Step 1: Design for maintenance
4.5 Detailed design of parts
Detailed design of parts is the fourth step in detailed design phase in the design development process.
During the design of an individual part, its specific location and function in subsystem in relation to fit in overall system goal needs to be considered. No questions pertaining to its design should remain unanswered, and any ambiguity about its shape, its material, its surface treatment and interfere with other components it interact should be answered. At this stage from the abstract to the concrete, from the concept of the system or device to the physical embodiment, the final transition is made and the idea merges into physical reality. At present, great progress has been made in part design, particularly through sophisticated methods such as photo elasticity and the finite element methods.
It was decided during conceptualisation stage of the newly designed tricycle rickshaw (Chapter 3, Section 3.6.2 Identification and formulation of the design problem, p 89) that possibility of outsourcing standard parts and components for the rickshaw from the readily available parts / components bins from existing bicycle and tricycle industries is to be explored (Table 3.2 and 3.3, p 71), and only elements would be developed that is not readily available in market. Once these parts and components were procured, the task of the detailed design of parts gets confined to the body shell and its related parts for integrating the different sub-systems into a total system to obtain a tricycle rickshaw as designed and conceived. Other accessory parts and components were designed include seat, seat back, hood, floor boards, mud guards, rain guards, side panels/
guards and rear panel etc. In the interim version (Fig. 5.27, p 161), many of these items were designed using traditional skills of the persons in trade. However in the Dipbahan+
version (Fig. 5.35 and 5.36, pp 168-169), most of these except the rain guards were replaced with Fibre glass Reinforced Plastic (FRP) components (Table 4.2, p 142).
Various steps taken for fabricating the above mentioned parts are given below:
1. Pattern making using plywood, wood and galvanized sheet.
The first step in making a Fibre Glass Reinforced (FRP) component is fabrication of the pattern. Pattern is the exact replica of the component to be made in FRP for the face side, made in materials that can be easily curved. Based on the expertise available, it can be made in plaster of paris, Mild Density Fibre (MDF) board or in wood suitable for curving. Plaster of paris patterns are relatively cheaper, easier to make but not reusable.
Thus when there arises a need for modification of the pattern, fresh patterns are to be made once again. Thus plaster of paris is not always preferred. Medium Hard wood used for furniture is good for this purpose. Also based on the type and size of the pattern, combinations of various other materials can be used for this purpose. Whatever be the medium of making the patterns, these are finished with automotive grade putty to cover any irregularity and obtain a smooth finish required for the final components. Surface finish that is desired in the final component is dependant on the surface finish of the pattern. Normal way of finishing the pattern is to first apply putty with spring plates, then finishing this with emery paper upon drying and then applying putty through spray painting. This is finished through wet sanding to obtain a smooth and fine finish. Finally wax based polish like waxpol is applied to obtain a fine glossy surface. The pattern must be resistant to water and one of the reasons for applying automotive grade putty is also to make this pattern water resistant. Most important point requiring attention is avoiding any under cut in the design of the components and providing appropriate draft angle for ease of removal of the components from the moulds. Normally at least 1.5° is provided as draft angle.
2. Mould making in FRP.
The second step for making FRP components is fabrication of the moulds. The mould itself is fabricated in FRP. The process of mould making starts after pattern is ready. The pattern is to be coated with releasing agent. In this case Polyvinyl Alcohol (PVA) is used as releasing agent. PVA is normally available in white crystalline granules. It is boiled in water at 1:10 solid to liquid ratio and after cooling the solution, it is applied with a brush or a sponge onto the pattern and dried in shade to obtain a fine coat of film. Usually two coats of PVA is applied.
After the pattern is coated with releasing agent, it is ready for application of FRP materials. Normally after releasing agent, a layer of gel coat is applied. Gel coat is either polyester or epoxy resins with slight difference in viscosity etc. It is called as gel coat, because, pigments for colouring the components are mixed in this type of resin and are applied before glass fibres are laid and impregnated with resins. In case the final FRP components are to be made in lighter colour, the mould is prepared in dark colour, so
that any irregularity in application of the Gel coat for the components will be easily visible. The colours are reversed for mould and components when final components are in dark colour. In case of the tricycle rickshaw, the colour of the final components are in Post Office Red and Golden Yellow colours. Therefore, moulds were made in dark colour, actually in black colour. Gel coat for application is prepared by mixing black pigments and for setting it, catalyst and accelerator were mixed with it, their percentage being 2% by weight. Accelerator used is Cobalt Octate. For FRP 2 % solution of 2%
strength solution is used. Accelerator is uniformly mixed prior to catalyst with the gel coat or resin to obtain uniform and gradual polymerization. However, polymerization starts only after mixing the gel coat or resin with catalyst. Catalyst used for FRP is Methyl Ethyl Ketone Peroxide (MEKP) 2% of 50 % solution strength is used. This gel coat mixed with accelerator and catalyst gets set very quickly and hence required to be applied fast. Gel coat is applied with a brush onto the pattern and allowed to dry. Once dried, this is ready for application of fibre glass and its impregnation with resin prepared same way as that of gel coat except mixing pigment.
The glass fibre is available in different forms. For making mould, non-woven mats with randomized direction of fibre is suitable. Glass matt of 400 Grams per Square Meter (GSM) is used for making the mould and 2 layers were applied. Once glass fibres are laid and impregnated with resin, it is allowed to dry. Once setting has completed, the mould is separated from the pattern. However, to prevent any warpage and damage to the mould during component making, the mould is reinforced further by adding wooden batten at the back side of the mould and fixing these with embedding in resin and glass fibre. The mould after separation from the pattern needs polishing before a component can be taken out. This is done by applying rubbing compound and polishing the inner side of the mould to obtain a fine and smooth polish. After the polish, a coat of wax polish is applied on to the mould.
3. Component making out of the mould.
The final step is making of the components using the moulds. This is exactly the same as making of the mould. First, a coat of release agent i.e. PVA is applied on to the surface of the mould. This is dried and Gel coat mixed with required colour of pigment is applied on the mould. Once gel coat is applied, it is dried. Care must be taken to see that whole surface is uniformly coated to avoid any unevenness in colour of the component.
Once the gel coat is dried, glass fibre mat is applied and impregnated with resin. For the final component, a surface mat with smooth surface and low GSM is first laid and then based on the size and type of component, either one layer of 400 GSM mat is used or one layer of 200 GSM mat followed by another layer of 400 GSM mat is used. For superior surface finish at the reverse side of the component, a fine surface mat of low GSM can be used. Once the resin is set, which normally takes 3-4 hours with 2% each of catalyst and accelerator mixed with it, the component can be taken out and process repeated for more components.
Table 4.2 Fibre Glass Reinforced Plastic components for Dipbahan+ (Fig. 5.35, 5.36, p 168-169) Name of
Component, its dimensions and
weight
Pattern Mould FRP Component
Hood [Length xBreadth xDepth]
(L x B x D) 1,350 x 850 x 250 mm, Weight 5.500 kg Back Panel (LxBxD) 875 x 825 x 50 mm
Weight 3.000 kg
Combined seat (LxBxD) 830 x 550 x 530 mm weight 4.500 kg
Floor board- front (LxBxD) 895 x 385 x 50 mm
Weight 3.000 kg Floorboard-rear (LxBxD) 895 x 485 x 35 mm
Weight 2.500 kg Mud guard Left & Right (LxBxD) 800 x 350 x 110 mm 2.000 kg each Side panel- Left
and Right
(LxBxD) 460 x 405 x 45 mm 1.500 kg each Total weight of all components 25.500 kg