English
中文RESUME
The paper discusses the machining process for a connecting rod of a passenger car and the main bearing of a horizontal wind turbine of 2 MW capacity. For each component, details such as a brief introduction to the product, its use, material used for the component, machining sequence, machine details and pictures and the cutting parameters are given. Other details given include the quality aspects and the tooling required for the part.
PART 1: THE CONNECTING ROD FOR A TYPICAL PASSENGER CAR ENGINE
The machining process of a connecting rod is given in this section.
1.1 Introduction
The Connecting Rod or Con Rod as it is called is a key element in the reciprocating IC engines of passenger cars. It links the crankshaft with the piston and converts the linear motion of the piston into rotary motion of the crankshaft. The connecting rod has a long and slender shape with a big end and a small end. The big end fits over the crank pin of the crankshaft while the small end fits on the piston pin of the piston. Crankshafts are made of forged steel for higher power requirements and from Aluminium forgings and castings for smaller engines, pumps and other reciprocating motion devices (Duffy, 1994).
1.2 Product Research
The main function of the con rod is to transmit the motion and force from the piston to the crank pin. There are a number of forces that act on the con rod they are: force from the gas pressure and inertia of the piston; force due to friction of piston and piston rings when it rubs against the wall of the cylinder bore; connecting rod and crankshaft inertia and the friction in the small and big ends. To a certain extent, using a bimetallic bearing that is press fitted and fine bored in the two ends reduces the force in the big and small ends. In addition, a deep oil hole is gun drilled along the length of the con rod for lubrication and these open in the big and small ends. Additionally, the bimetallic bearing would have oil grooves that are used to circulate the lubricating oil on the piston pin at the small end and the crank pin at the big end (Timoshenko, 1997).
Cross section of the con rod can be given a cross section of circular, rectangular, I or H sections and also tubular when weight is an issue. For slower engines, a circular section can be used while for medium to high speed engines, as in passenger cars, an I section can be used. The length of the con rod to the crank pin radius ratio is maintained at 4 or 5 (McFarland, 1999).
Maximum force due to gas pressure Fa = π d2pe/4
pe is the explosion pressure.
Force from the piston inertia Fi = Mw2r (cosθ+r/l cos2θ)
M is the mass of piston and rings+ piston pin +1.3 of the connecting rod)
= 4.14 x 106 d3 Kg for cast iron pistons
and 1.78 x 106 d3 Kg for aluminium pistons
d is the more dia in mm
w is the angular speed in radians/ sec
r is the crank radius in mm
l is the con rod length in mm
1.3 Material Selection
Connecting rods are subjected to immense bending stress and elongation stress. They would be operating in the engine cylinder where combustion and firing is taking place. The material that is selected should be capable of withstanding very high stress and temperatures and should show high hot hardness, ability to withstand alternate bending and elongation forces should be torsionally stable. The material should have a high fatigue resistance not deform under these conditions as any small distortion of failure due to frictional heat would mean a catastrophic failure of the engine and can result in death of car passengers or bystanders. Ideally, the following composition of the alloying elements is required (Timoshenko, 1997).
The following types of steels can be considered for the material: 40C8, 37 Mn6, 35Mn6Mo3, 35Mn6Mo4, 40Cr4, 40Cr4Mo3, 40NiCr4Mo2 and other types of similar steels. While forging is the most economical method of preparing con rod raw stock, powdered metallurgy by sintering can also be considered but the costs are high and suitable only for smaller con rods (Timoshenko, 1997).
1.4 Product Analysis and Outline Process Choice
The sub components of con rod can be seen in ‘Fig 1. Connecting Rod Part Drawing’. The main dimensions that would be machined are the big end and small end bores, side faces, slitting of big end to create the big end cap and two bores for bolt mounting and an central lubrication hole optionally gun drilled. References for each operation are given along with the machine picture.
1.5 Detailed Process Plans
The detailed process plan as per the above table is given for each operation number. Operation 10: In this operation, both side faces of the connecting rods, for both the big and small ends are milled. While this dimension is not very critical, the width has to be closely controlled so that the con rod sits properly in the crank pin without moving side ways. A good surface finish is also required so that the side thrust bearings and oil seals are not damaged on the crank pin. After milling, a light surface-grinding cut can also be taken to provide a good finish and to control the sizes. Pictures of the milling machine used for machining is as shown below along with the feeds and speeds.
A rough core drilling operation is performed on the big and the small ends, using two fixtures, mounted side by side. Location is taken by butting against the pick up point and resting on the milled surface. The small end is drilled and reamed first and then the bore is used for location in the big end drilling. The big end is then drilled. It is also possible to use an indexing fixture.
Operation 30: Semi Finish bore big end
Location is taken in the reamed small end bore on a fixture and the big end is first semi finish bored, keeping about 0.5 mm allowance for the finish bore. The operation is done to ensure that any distortions and out of roundness and centre distance are now maintained. Please refer to the following table for pictures of the machine and cutting parameters.
Operation 40. Mill, drill and tap Big End Top
This is an important step since the mounting hole seat milling, counter bore drilling, reaming and tapping will be done. It is best to complete all the operations in the same set up. A vertical CNC machine is proposed where first a side and face cutter will mill the seat for the mounting hole. Next drilling would be done for the tap size and then a counter sunk drill will be used to make the counter bore. If required, reaming of the counter bore and thread hole can also be done. Tapping as per the required thread size and diameter will be done at the last stage. Since there would be a slitting operation, the counter bore will extend till the parting line where the slitting will be done and extend slightly beyond so that there is no damage to the thread after the operation is over. Location will be taken in the small end of the con rod that has already been reamed.
Operation 50. Slit Big End
In this operation, the big end cap is slit into two parts on a vertical milling machine and by using a slitting saw. Slitting is required other it will not be possible to fit the con rod on the crankpin of the crankshaft. Please refer to the following table that gives details.
Operation 60. Punch serial matching numbers
The slit cap of the big end and the rest of the con rod are now a pair and must be always kept together. A running serial number can be punched on the top of the con rod and on the inside rib of the con rod.
Operation 70. Clean Deburr and Inspect
The pair of big end cap and connecting rod must be deburred with an air gun having steel mounted point to remove all sharp edges. All loose burr must be removed and the component washed in a degreasing and cleaning machine using non rusting solvent and water. The parts will now be ready for inspection such as 100 percent crack test and random sampling of other dimensions.
Operation 80. Assembly
In this operation, the big end is assembled with the con rod by using mounting bolts. A special fixture is used to hold the con rod in place. A torque gun has to be used to provide the required amount of torque while tightening the bolts. The bolts have to be tightened partly, then loosened and then again tightened, so that residual torque is removed. Some con rod manufacturers for larger vehicles may also assemble thin wall bi metallic bush bearings inside both ends by using a press machine. If these bearings have to be fitted, then for the big end an additional notching operation must be carried out in operation 50, for both the con rod big end and the cap. The notch retains the bearing in the component.
Operation 90. Finish Bore Big and Small Ends
This is the final operation of boring for both the big and small end. A light cut of 0.1 mm may be taken on a horizontal or vertical boring machine. First, location is taken in the small end bore and the big end is bored. Then location is taken in the big end and the small end is bored. All boring has to be done in the assembled stage only, even if a bi metallic bearing has been fitted in the two ends.
100. Clean Deburr and Inspect and pack
The con rod assembly has to be deburred and cleaned thoroughly to remove all grease and loose burr. The component can then be inspected for centre distance, bore sizes, surface finish of bore and width. Crack testing has also to be done. When all operations are over, the con rod is packed to prevent dents and rusting and sent to the assembly shop.
1.6 Quality Aspects
Connecting rods are very important components that are subjected to intense stress and strains. When the engine is running, if the connecting rod fails, then it can result in bursting of the engine block along with injury to personnel standing near the vicinity. The product is relatively simple and there just a few dimensions that have to be maintained. Important dimensions are the centre distance between the big and the small end, bore diameter of the big and small ends, roundness and concentricity of the bores, surface finish of the bores, flatness of the end faces, surface finish of the bores and the mounting threads sizes. These can be maintained by using proper SPC charts to understand how the size variation occurs and how the machining parameters can be controlled. The Cp ad Cpk should be above 1.67 for the process to be stable.
The above process can be used for low volume production. However, when high volume production of more than 500 components are required each day, then a production line with special purpose machines, machining centres and multi spindle machines have to be used.
PART 2: THE MAIN BEARING OF A 2 MW DIRECT-DRIVEN HORIZONTAL AXIS WIND TURBINE
The machining process for the main bearing of a 2 MW direct-driven horizontal axis wind turbine: the bearing between the rotor and the generator is given in this section.
2.1 Introduction
The bearings of the wind turbine are subjected to severe and fluctuating loads the vary between high load and low speed, high load high-speed conditions. The turbine blades are subjected to up thrust and down thrust wind force and so the bearing will be subjected to alternate axial and radial loads. These forces tend to pull the main shaft either out of the bearing housing or push it into the bearing housing and in addition, the bearing may be subjected to electric arcing and lightning strikes of a few million volts of electricity. While problems of electric arcing and lightning can be eliminated by proper grounding, this becomes a problem in offshore wind turbines. Any failure of the bearings can lead to catastrophic failures of the wind turbine and lead to death or injury of bystanders, who would perhaps be subjected to a high-speed blade slashing at them (Mathew, 2006).
2.2 Product Research
A proper bearing will help to eliminate most of the above problems and also provide a smooth drive for the main shaft. A 2 MW wind turbine would generate about 2.5 million Newton meter of torque at a speed of 1800 rpm. In addition, wind turbines are installed in far off places and the area where the bearing is mounted is often difficult to access. Therefore, maintenance of the bearing along with lubrication is a key issue. What is required is a bearing that would require very low maintenance, have a longer life and one that would not generate much heat and be able to withstand heavy impact load with alternate radial and axial loads (Mathew, 2006).
Keeping in mind all these conditions and according to the recommendations made by SKF, Spherical Roller Bearings are recommended for the main shaft support that runs between the rotor and the generator. As a safety precaution and to prevent shaft buckling, two bearings are used for the support for the main shaft (SKF, 2009).
It must be noted that these are high precision bearings and have to correspond to the specifications made by American Bearing Association. Such bearings need to conform to certain grades such as 10, 20, 25 and so on and a higher number means a cheaper bearing with lower quality. Since the discussion is for a 2 MW wind turbine, bearings made by leading organisations such as SKF, NTN, FAG and other companies are recommended. While there are a number of bearing manufacturers, the material consistency, quality, heat treatment and manufacturing may not be very reliable. Any failure in the bearings is catastrophic and would bring down the while wind turbine and the cost would be much more than what a high quality bearing from SKF or FAG would cost (Shigley, 1990). Two views of the bearing are shown in the following figure.
Since the bearings would be exposed to high intensity winds with dust and water, it is proposed that sealed spherical roller bearings should be used. Seals are provided by fitting a thin cover of resin-based polyester that is water resistant and will not contaminate the oil. In addition, any grease that is applied for lubrication will be retained in the bearings and the seal can be removed and replaced when fresh grease has to be applied. The bearings also have a minimum of lip face so that they can be assembled in restricted spaces. Please refer to the following figure for details of the bearings (SKF, 2009).
2.3 Material Selection
The bearings need to resist abrasion and friction and be able to withstand impact loads caused by wind speed changes. Corrosion Resistant hardened steel of grades AISI/ SAE 440C, 440B, 420, 410 or 329 can be used for the balls and races. It is also possible to use sintered silicon carbide balls as these generate very less heat but they are very expensive and may not be suitable for the impact loads. In hot areas such as deserts, where the ambient temperature is high and heat is not dissipated, Chrome steel of grades such as AISI/ SAE E52100, E51100 can be used. To reduce the costs, steel of grade such as AISI/ SAE M50, SAE 1008 and others can be used. SAE 52100 is also used for Races. Bearings are subjected to intensive compressive loads and hence they should be carburised hardened to 60 Rockwell and precision ground. Full hardening is not recommended as the softer core would absorb the load and prevent the balls and races from cracking (Timoshenko, 1997).
2.4 Product Analysis and Outline Process Choice
The spherical roller bearings will have a number of sub components that are produced individually and then assembled. The bearings are made of: inner and outer races and the inner race will have a seat ground on the outer diameter while the outer race will have a seat ground on the inner diameter; cage in which the rollers are housed and rollers. The process of bearing manufacture is briefly explained as given in the following table. References for each operation is given along with the machine picture.
2.5 Quality and Mass Production
Bearing quality and reliability plays a very important role in the life of the windmill. Quality begins with maintaining the correct hardness as specified and if the component and sub component is too hard, then the bearing will either shatter or fail and if it too soft, then the bearing will not be able to sustain the high loads. Surface finish and play between the balls and the races is important. If the surface finish is too rough, then the bearings will not run true and there will be an excess of run out (Shigley, 1990).
The method mentioned above is outlined for mass production. By adding CNC machines, it would be possible to increase the output.
