Group 2 Computer Automated Design

Seth Peter Karpinski Design Project

Jason Howard Lichtman Professor Ateshian

Rena Miriam Rudavsky Final Report

Erol SearfossNovember 12, 2005

Computer Aided Design Project

CAM ROLLER-FOLLOWER VALVE DESIGN

Air Valve System Design

Design Objectives

  • Design air valve system to drive compressed-air piston engine
  • Valve must be mounted on existing engine with no engine modifications
  • Valve must use compressed air supply in ME Lab (max. 80 psi)
  • Create design using CAD software, including animation of valve mechanism, detailed 2D drawings, and bill of materials
  • Evaluate engine performance

Criteria for Valve Design Evaluation:

  • Smallest number of parts (custom and off-the-shelf)
  • Manufacturability and cost of labor
  • Robustness of valve design performance to machining tolerances
  • Size of parts
  • Cost of stock materials and off-the-shelf components
  • Safety, reliability and maintainability

Preliminary Design:

Smallest number of parts: This design is relatively rudimentary; the parts included are the cam, follower, spring, housing, valve fittings, and bearing. By integrating the linear slide directly into the shaft, the design the assembly is simplified. To avoid excessively complicated part fittings, the cam-follower spring is located on the back of the housing. One of the paramount advantages to this particular design is its simplicity.

Part / Qty
Cam / 1
Bracket / 2
Bearing / 1
Follower / 1
Housing (right) / 1
Housing (left) / 1
Pin / 1
Spring / 1
Total / 8

Manufacturability and cost of labor: The assembly manufacturability is quite easy; all parts can be created with the cnc or purchased over the counter. The cam is clearly the most intricate part to design as well as to machine.

Robustness of valve design performance to machining tolerances: The valve design ideally would allow the air to be injected through the valves at constant flux. But, because the valve opens and closes gradually, as opposed to instantaneously, our design is not theoretically ideal. While our design is not perfect, it is still effective. And it has the added bonus that upon examination of the necessary tolerancing considerations, (specifically of the inlet system and the disk of the cam) the design provides a cushion for reasonable error. The surface area between the shaft and the housing through which air can pass appears to be relatively minimal compared to other considered designs.

Size of parts: At this point in the design dimensions are still being finalized pending decisions on spring and cam size optimization, and final follower shape.

Cost of stock materials: The housing, cam, and follower will be fabricated from machined aluminum. Additionally, a simple, inexpensive rollerblade bearing will fulfill the goal of frictionless contact with the cam. Unfortunately, stock parts for the chosen materials are limited, if not entirely non-existent. Thus, the cam, housing, and follower must all be custom machined.

Safety, reliability and maintainability: Safety considerations for the particular design do not exceed reasonable expectations. A housing enclosing the follower limits any unnecessary injuries that may be incurred as a result of fast moving parts. Also, the small number of parts constructed of durable material increases the reliability and decreases the necessary maintenance.

Description of preliminary design

Our design is based on the roller follower valve system. The system uses a traditional housing, follower, and cam. The housing will be directly attached to the ground for stability. While the exterior of the housing is square, the interior houses a cylindrical hollow. There are two holes on the left side of the housing that connect with tubes to the piston engine. The opposite side of the housing contains an ellipse-shaped opening that permits the pressurized air input to travel from one output outlet to the other.

The interior cylindrical hollow within the housing provides a constricted linear guide for the follow. The follower is machined to permit free horizontal movement while minimizing air leakage. Three holes drilled into the middle of the follower channel the pressurized and atmospheric air input into the respective outputs, a bearing attached to the follower via a bracket and pin maintains direct contact with the cam at all times. The bearing reduces the amount of friction between the cam and follower. A spring forces constant contact between the bearing and cam as it pushes against the housing. As the cam rotates it pushes the follower from the starting position in which pressurized and atmospheric air enter through the first two inlet holes to position 2 where pressurized and atmospheric air enter through the last to inlet holes. The compressed spring then pushes the follower back to the starting position switching pressurized air input, thus completing one cycle.

How it meets design objectives:

The described system is designed to drive a compressed-air piston engine. The valve is designed to permit mounting on the existing two-cylinder engine with no engine modifications. Air pressure of up to 80 psi supplied in ME lab is more than sufficient to drive this valve system. We have preliminary CAD drawings that approximate our system and are prepared to make them more detailed as we fine-tune our design. Engine performance will be evaluated during testing.

How it works:

Compressed air is the source of energy for this system. Our valve from starting position channels compressed air to the appropriate side of the air-piston. While one side of the piston is filling, it is turning a crankshaft. This shaft is connected to a cam that moves the valve to channel compressed air to the opposite side of the piston. This cycle repeats and runs the engine. Our valve system is a roller follower system. The cam to be designed will output a dwell-rise/dwell-fall pattern. With this pattern there are two main positions for the follower. The follower supplies air to one side of the piston during the first dwell, switches quickly to the other side during the rise section, then supplies air to the other side of the piston during the second dwell. The follower will then return quickly to its initial dwell position during the fall section of the pattern. Compressed air is connected to the middle of the three holes in the follower. As the follower moves in and out of the housing the compressed air is directed into one of the two holes in the left side of the housing or the other. The follower has 3 holes because as the compressed air follower hole lines up with one hole in the housing, one of the three holes in the follower lines up with the other hole in the housing. This allows air to escape the piston engine to atmosphere.

Final Design:

Smallest number of parts: This design is relatively rudimentary; the design includes five partially or fully machined parts plus nuts and bolts: a cam, follower, housing,back bracket and the support bracket. These custom parts are supplemented by the spring, bearing, clevis pin, and nuts and bolts. While five parts may seem somewhat excessive, a balance was struck between ease of machining and number of parts. The particular design puts a greater emphasis on ease of machining. Additionally, while a bracket and back-plate could have been purchased, the cost/benefit ratio for strength, durability, and flexibility resulted in their in-house fabrication.

Manufacturability and cost of labor: One of the paramount benefits of the proposed design is the straightforward custom machining. The current design requires modification of a purchased rectangular bar to create the follower. The cam, housing halves, and brackets are manufactured using Pro-E aided CNC machining. All parts except for the support bracket can be machined from one position in the CNC. This is key- the process of manufacturing involves the relatively simple steps of Pro-E coding, mounting, and machining.

Unfortunately, the support bracket involves two mountings in the CNC machine. The cam must be separated from the larger block of mounting aluminum, and the set screws holes must be drilled.

To ease machinability, the two halves of the housing were designed to be machined at the same time out of a larger block of aluminum assuring near perfect line-up tolerancing. This longer piece can then be cut into two, providing the two halves. All holes in both sides of the housing are referenced off of one point- again increasing our ability to reach desired tolerances. Essentially, the necessary machining is surprisingly minimal. Since custom machining is nominal cost of labor is low.

Robustness of valve design performance to machining tolerances: Ideally, the valve mechanism would instantaneously switch the pressurized air and atmospheric input into the piston, allowing a constant flux of pressurized air. However, because the valve opens and closes gradually, not instantaneously, our design is not theoretically ideal. While our design is not perfect, it is still effective. The current dimensions of the design represent a trade-off between two of our biggest considerations: air leakage versus friction at the interface of the housing and follower. If there is enough space for the follower to move freely, there will be air leakages. Thus, tolerancing considerations are of utmost importance. Due to our group’s limited machining experience, it seemed logical that we limit manufacturing. Purchasing the shaft via McMaster allows the design group to concentrate solely on tolerancing of the housing. While there is no such thing as a perfect fit between the follower and the housing, the beauty of the current design revolves around its simplicity regarding tolerances. With bought parts, the only issue is that the vertical sides of the follower must lay flush against the vertical interior sides of the housing, which is only an issue of flatness. The horizontal distance between the two halves of the housing can be adjusted along a threaded bolt, thus assuring the proper trade-off between air leakage and friction after testing. And, since the custom machining is surprisingly minimal, the robustness of the design is greatly increased.

Size of parts: While the size of the valve system is roughly, length 5 inches, width 1.7 inches, and height of 1.75 inches (without bracket). The size of the cam is of particular interest. With three variables for cam design, we decided that our goal for the cam is to minimize the beta value and keep a relatively small concavity. The lower the beta value, the closer the engine performance will be to ideal. The roller follower permits a cam with a slight concavity but a large concavity causes the cam to catch on the follower. The design minimizes the beta value by utilizing a fairly substantial length of roughly 2.5 inches. This large cam length, however, results in some vibrations during operation. Again like other issues of the valve design, tradeoffs exist. By making the cam larger to minimize the beta value, the risk of a certain degree of operational stability presents itself.

Cost of stock materials and machining costs: All parts were purchased or priced through McMaster. Large quantities of stock, nuts, and screws were priced in order to minimize unit costs. The housing, follower, and springs were actually purchased; therefore bulk purchasing was not taken advantage of. If materials were purchased in bulk the unit cost of the respective pieces would be lower. Below is a summary of stock costs as well as machining costs.

Safety, reliability and maintainability: Safety considerations for the particular design do not exceed reasonable expectations. A housing encloses the follower to limit any unnecessary injuries that may be incurred as a result of fast moving parts. Also, the high number of stock parts constructed of durable material increases the reliability and decreases the necessary maintenance.

Description of preliminary design

Our design is based on the cam roller-follower valve system. The system uses a traditional housing, follower, and cam. The housing will be directly attached to the base for stability. The exterior of the housing is square; the interior houses a series of steps forming a complex rectangular cross section. Although complex in appearance, these steps can be machined easily. The steps in the interior significantly decrease friction between the housing and the shaft. There are two holes on the left side of the housing that connect with tubes to the piston engine. The opposite side of the housing contains an ellipse-shaped opening that permits the pressurized air input to travel from one output outlet to the other.

The interior rectangular hollow within the housing provides a constricted linear guide for the shaft. The shaft is machined to permit free horizontal movement while minimizing air leakage and friction. Three holes drilled into the middle of the shaft channel the pressurized and atmospheric air input into the respective outputs. A bearing attached to the follower via a clevis pin. The follower maintains direct contact with the cam at all times with the help of a spring which pushes against the back of the housing. The bearing reduces the amount of friction between the cam and follower. As the cam rotates, it pushes the shaft from the starting position in which pressurized and atmospheric air enter through the first two inlet holes, to position 2 where pressurized and atmospheric air enter through the second and third to inlet holes. The compressed spring then pushes the follower back to the starting position switching pressurized air input, thus completing one cycle.

How it meets design objectives:

The described system is designed to drive a compressed-air piston engine. The valve is designed to permit mounting on the existing two-cylinder engine with no engine modifications. Air pressure of up to 90 psi supplied in ME lab is more than sufficient to drive this valve system. We have preliminary CAD drawings that approximate our system and are prepared to make them more detailed as we fine-tune our design. Engine performance will be evaluated during testing.

How it works:

Compressed air is the source of energy for this system. Our valve from starting position channels compressed air to the appropriate side of the air-piston. While one side of the piston is filling, it is turning a crankshaft. This shaft is connected to a cam that moves the valve to channel compressed air to the opposite side of the piston. This cycle repeats and runs the engine. Our valve system is a cam roller-follower system. The cam to be designed will output a dwell-rise/dwell-fall pattern, resulting in two main positions for the follower. The follower supplies air to one side of the piston during the first dwell, switches quickly to the other side during the rise section, then supplies air to the other side of the piston during the second dwell. The follower will then return quickly to its initial dwell position during the fall section of the pattern. Compressed air is connected to the middle of the three holes in the follower. As the follower moves in and out of the housing the compressed air is directed into one of the two holes in the left side of the housing or the other. The follower has 3 holes because as the compressed air follower hole lines up with one hole in the housing, one of the three holes in the follower lines up with the other hole in the housing. This allows air to escape the piston engine to atmosphere.

Alterations from Original Design:

The primary alteration made on the design was the shape of the shaft and housing. The initial design featured a cylindrical shaft set within a two piece housing. The final design features a rectangular shaft set in a three piece housing. The advantages of the new design include a more restricted horizontal motion, reduced housing-shaft friction, and integration of over the counter parts. The cylindrical shaft had no means of restrict rotation; however the geometry of the square shaft restricts shaft rotation. The square shaft is purchased from McMaster, toleranced to a high degree. The new housing consists of a right, left, and rear piece. The housing is manufactured in such a way that the shaft is in contact with only the vertical walls and a small portion of the horizontal walls within the housing. This change allows for reduced friction of the shaft/housing interface. The back of the housing has also been altered. The original design featured a closed back which was integrated on the rear of the left and right housing. The final design features a separate back. The back must be attached in such a way which allows placement adjustment to account for the aforementioned housing flexibility.

The roller-shaft connection has been altered from the initial design as well. The original design featured a bracket for the roller-shaft connection. The new design integrates the connection directly into the shaft. This reduces the number of parts utilized but increases machining costs. The bearing itself was also altered. The initial design called for a simple roller blade bearing. Following research a bearing with a smaller diameter was selected. The reasoning being that if the cam was concave the bearing would be able to follow more closely.