Cams and Followers(3)

Construction of a Cam Profile for the Radial Cam

To draw the cam profile for a radial cam first of all the displacement diagram for the given type of motion of the follower is to be drawn. Then by constructing the follower in its proper position of the each angular position then the profile of that working surface of the cam is drawn. In constructing the cam profile principle of kinematic inversion is mostly used i.e. the cam is imagined to be stationary and the follower is allowed to rotate in the just opposite direction to that cam rotation.

Manufacturing considerations

• Materials: Hard materials as high carbon steels, cast iron. Sometimes made of brass, bronze and plastic cams (low load and low-speed applications).
• Production process: rotating cutters and Numerical control machinery. For better finishing, the cam can be ground after milling away most of the unneeded material. Heat treatments are usually required to get sufficient hardness to prevent rapid wear.
• Geometric generation: actual geometries are far from been perfect. The cycloidal function can be generated. Very few other curves can.

METHODS OF CAM PRODUCTION:

1) CASTING:

Chilled cast iron is primarily used for the production of the cam shaft. The development of an automobile industry and engine power brings up more advance requirement for properties of the camshafts. In the casting process, there are many possible of casting defects such as shrinkage defect, porosity, crack and the insufficient pouring. However for more and higher loads in roller contact, cast camshafts with induction hardened cam lobes can be used.

2) FORGING:

Manufactured or Forged from steel bar camshafts are also used for certain high loaded diesel engines. These are produced on computer-controlled forging systems with integrated heat treatment or machined from steel bar.

3) MACHINING:

Machining is necessary for derived final dimension to the cam.

1. While using Casted or Forged CAM only Final Machining is require achieving final size within the required tolerances. In this process casted or forged CAM are in the required shape but their size is maintained by machining and finishing operation.
2. While Using Metal Billet (metal rod), the profile of cam is obtained by removing excess material which from metal billet. In this process, only machining is required to manufacture CAM.

A detail description of the processes for mass manufacturing is as follows:

• TURNING & DRILLING: The raw and forging is put in this machine and center drilling and turning on one side is done.
• TURNING: The turning of the six journals takes place. There are four tools used for this turning – Rough, Neutral, and Left and Right tool. Also, Grooving and Parting operation is performed on the left side of the shaft.
• DRILLING: This machine drilled the diameter 4.5 dowel hole which is used as a reference for further operations.
• GRINDING JOURNALS: The grinding and finish grinding of the six journals takes place. Carborundum wheels are used for grinding.
• GRINDING ON FACE: An angular grinding wheel is mostly used for the face grinding operation. At this stage, inspection is done after every ten components using gauges.
• DRILL DOVEL HOLE: Grinding of the Cam is done here. The dowel hole is taken as reference.
• LAPPING: To give the super finish in microns, lapping is done using lapping paper on the Cams and Oil seal area.
• SLITTING: This machine made a slit on the right side of the shaft to fit in the engine. The cutter used as a saw cutter and pneumatic deburring is done here.
• AIR & WATER CLEANING: This is the Washing Machine where water and air jets are used to clean the component of dust, oil, chips etc.
• INSPECTION: This is a Measuring Machine used to check for tolerances - Run outs and Diameters of journals all six and center four. The machine then declares the component as OK, NG or BAD.

Analytical Cam design

In the cam design, the graphical method is replaced by the analytical or mathematical method. This process has been supported by the digital computer which provides much advantage than graphical method. First of all, once the computer programmed it can generate many different cam profiles and design information in the short interval of time than graphical cam layout in drawing board. Also, a number of a trial can be done.

It has high numerical precision and design shapes which obviously help in design procedure, smoothly and dynamic operation. It provides necessary information for manufacturing the cam. Mainly CAD and CAM software are used for analytical design. CAD/CAM techniques (Computer-Aided Design/Computer-Aided Manufacture) have been increasingly used in industry as technology has become advanced. Traditionally the designing would be done by hand and a workforce would be required for manufacture. The products can now designed and manufactured much quickly through CAD/CAM and models and prototypes can be made quite easily by rapid prototyping techniques.

Lift Curves

The purpose of the cam lobe is to raise the lifter and open valve. We can look at the lobe but it doesn’t tell us exactly how it is going to do its job fluently. The lift curve is a much precise way to look at that cam lift. It is a graph of the lifter or valve motion as the cam rotates. The lift curve can be measured by using a degree wheel and the dial indicator or more accurately using a computer driven cam profiling system. The opening of intake ramp and the flank with the intake nose are indicated on the graph. The ramp does not extend much beyond when the valve opening usually less than 0.015 in (0.4 mm) lift. After the ramp, the large upward curvature shows the start of the flank. The portion of the nose is the large central area with negative curvature.

Fig: lift curve

References:
1. H.H. Mabie and C. F. Reinholtz, “Mechanism and Dynamics of Machinery”, Wiley.
2. J.S. Rao & R.V. Dukkipati Mechanisms and Machine Theory, New Age International (P) Limited..
3. J.E. Shigley and J.J. Uicker, Jr., “ Theory of Machines and Mechanisms”, McGraw Hill.
4. B. Paul, “Kinematics and Dynamics of Planar Machinery”, Prentice Hall.
5. C. E. Wilson, J.P. Sadler and W.J. Michels, “Kinematics and Dynamics of Machinery”, Harper Row.