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Machining of the crankshaft

Theory, measuring, specifications.

Crankshaft is a part of the crank mechanism designed for conversion between reciprocating piston motion and rotational shaft motion.

Crankshaft and its components

The crankshaft consists of:

  • main journal (shaft support in the main bearing);
  • connecting rod journal (shaft support for bottom the connecting rod journal);
  • webs (cross-over between the main and connecting rod journals);
  • shaft front extension (part of a shaft on which the camshaft drive gear wheel, generator power take-off pulley, and various attachable equipment are mounted);
  • counterweights (crankshaft components that provide main bearing relief from inertia centrifugal force, shaft balancing element).

Crankshaft measuring is performed in two steps:

Step 1: crankshaft inspection for main journal axis bending. The crankshaft is installed on rams. Use a dial gauge with a support to check the crankshaft axis bending, usually on the central journal first, by rotating the crankshaft. Bending shall not exceed:

  • for light engines — 0.005 mm.
  • for truck engines — 0.01 mm. (or see ICE manual)

Crankshaft axis bending measurement

Step 2: use a micrometer to measure the main and connecting rod journal diameters in turn as shown in the figure below, measure the main (Rm) and connecting rod (Rr) journal fillet radii using a radius gauge, and the crank radius (R) using a special measuring instrument (height gauge or beam caliper ShTs-150), see figure below. Crank radius is calculated using the following formula: R= {(Фr-Фm):2}+L; where:
     R is the crank radius;
     Фr is the connecting rod journal diameter;
     Фm is the main journal diameter;
     L is the distance from the main journal surface to the rod journal surface;

Measuring main and connecting rod journals A, B: diameter measuring locations along X and Y axis. (Main journal measuring is not shown for clarity)

Measuring main and connecting rod journal radii. 1 – crankshaft; 2 – radius gauge

After visual inspection and measuring of the main crankshaft geometrical parameters, magnetic testing or dye penetrant testing is performed to detect microcracks or chips.

Magnetic testing is carried out using a CD15 magnetoscope. Dye penetrant testing is carried out using red dye penetrant, cleaning agent and developer solution. Before magnetic testing clean the crankshaft surfaces from dirt and other deposits, degrease. Preliminary machining is necessary, e. g. adhered friction bearing residues must be ground away from crankshaft supports to get a clean journal surface.

When using dye penetrants (if there are any adhered bush particles, grind them away):

  • clean the surface with a cleaning agent;
  • apply penetrant and wait for a few minutes for it to fill the surface defects;
  • wash the penetrant off the crankshaft surface with a cleaning agent;
  • apply developer solution.

The defect will manifest itself by a dye penetrant on the surface covered with the developer solution.

Crankshaft journal surface roughness shall be not less than Ra 0.2 µm (according to the former classification, not lower than the Class 9). The same applies to axial play bearing seating.

Crankshaft levelling

Crankshaft levelling is performed when necessary. Cast iron crankshafts do not need levelling at all! Steel crankshaft levelling is performed carefully followed by journal and fillet crack inspection.

Correct repair is a transition to the next oversize without levelling!

There are several types of levelling: stress application across the axis, stretching or compressing crankshaft deformation areas.

Crankshaft levelling is performed by sustaining the radial runout in relation to the main journals, front and rear gland journals, journal under the GDM drive as well as in relation to the flywheel housing within 0.08–0.01 mm. Crankshaft straightening is performed using AZ СР 150 machine water press. However, you must remember that crankshafts that were straightened out in cold water can curve back again during the operation, therefore prior to levelling the crankshaft must be heated up to 100–125 °C (iron and carbon molecules do not change their structure under this temperature). When the crankshaft axis is straightened out, the axis is bent against the curve and then brought back to the radial runout limit up to 0.1 mm. Crankshaft levelling is performed when journal sizes exceed the crankshaft oversize after bending and when shafts are not loaded, i. e. the ICE is not turbocharged.

Crankshaft axis (main journal axis) straightening

After levelling it is also necessary to check the crankshaft for microcracks in the area of oil channel outlets, on fillets and journal surfaces that could occur during levelling. Magnetoscope or dye penetrant technology is used for this purpose.

Crankshaft grinding

Connecting rod journal wear in a circumferential direction is uneven. The journal body wears out more on the crankshaft axis side.

Gas pressure force transmitted through the connecting rod to the journal and inertia centrifugal force from the rod to the piston weight related to rotating parts impact the rod journal. Due to these forces, the journal is loaded and wears out on the crankshaft rotation axis side.

Worn out journals are repaired by grinding of connecting rod journals and main journals on a special AMC SCHOU К-1500 (Lshaft = 1,500 mm), К-1200 (Lshaft = 1,200 mm) and CG 360-3300 (Lshaft = 3,300 mm).

OD grinding machines equipped with bushes on main journals with centre holes displaced in relation to the main journal axis on the rod journal eccentricity value for crankshaft equilibration. In order to reduce the main and connecting rod journal axis misalignment, crankshafts are ground starting from rod journals, and the crank radius is strictly sustained (it doesn't change when passing to the next rod journal group). Rod journal grinding is different from main journal grinding in the number of shaft rotations chosen due to its length and weight. This is due to the fact that the crankshaft displaced from the rotation axis at a distance equal to the crank radius bends against the rotation axis. When installing connecting rod journals on AMC SCHOU К1500 and К1200 machines before grinding, the crankshaft is balanced with machine counterweights and is controlled by a pressure gauge of the machine headstock hydraulic drive. On the AZ CG360-3300 machine the control is carried out by an electric gauge. Tailstock tightening force can be adjusted by a hydraulic drive and is controlled by a dial gauge support which minimizes the crankshaft bending value by tailstock down pressure. Long crankshafts are ground with stays.

Crankshaft bending during rod journal grinding. 1 — machine centreline; 2 — centre displacement node to rod journal axis; 3 — crankshaft

When grinding crankshafts you must comply with the following specifications:

  • Main journal radial runout must not exceed 0.005 mm. Journal surface ellipse cone must not exceed 0.005 mm
  • Fillet radius is measured using a radius gauge and shall be not less than that for new crankshafts. It is formed when adjusting abrasion wheels with a special apparatus. Fillet surface roughness shall be not less than the connecting rod and main journal surface roughness and shall be equal to Ra 0.2 µm (not lower than Class 9 according to the former classification).
  • • Rod and main journal axis alignment shall be within 0.1 mm at a length of 1,000 mm (the parameter is sustained by equipment technological capabilities).
  •  Equal size of the crank radius (corresponding to new crankshaft parameters).
  • Main journal axis bending shall not exceed 0.005 mm for tractor unit crankshafts; quarry machinery ICE bending is according to the manufacturer's specifications.

With crankshaft axis bending of 0.08–0.40 mm, journals are ground to the following oversize: 0.25 mm; 0.50 mm; 0.75 mm; 1.0 mm (if oversize available. Otherwise the crankshaft is replaced).

After crankshaft journal grinding imbalance value is determined. If the imbalance standard is exceeded, crankshaft balancing is performed using special equipment.

Crankshaft balancing

After machining, crankshaft balance tolerance can be changed, therefore balancing is necessary.

First of all, crankshaft balancing must be performed. Then flywheel, pulley and clutch cover are installed on the crankshaft by turns, and balancing of these parts assembled on the crankshaft is performed.

Flywheels and clutches balanced separately by the manufacturer shall be balanced on the crankshaft only if their rotation axis was displaced during the main journal grinding.

Balancing is used when it is necessary to replace, for example, a flywheel balanced by the manufacturer within the crankshaft assembly, as well as in cases when it is impossible to find a correct relative position of the removed flywheel and the shaft that were balanced earlier because of disassembly by unskilled personnel.

Dynamic balancing of V-type engine crankshafts and in-line 2, 3, and 5-cylinder ICEs is especially difficult. Crankshafts of these engines do not have a centre medium plane, and some of them (for example, 2-cylinder engines) do not have a mass centre on the rotation axis. Such shafts are balanced dynamically only with special compensator weights on connecting rod journals to simulate reduced mass (in percentage terms) of bottom rod ends and keystone piston mass value.

Balancing is performed on a special НС-500 machine with accuracy up to 1 g/cm. The crankshaft is installed on separate rams that are connected by a link rod with an electronic assembly. The crankshaft gains momentum using an electric engine installed in rams, and the rotational dynamics is calculated and analysed by the computer.

The computer that stores the drill diameter and feeding angle values analyses the data and performs metal removal quality control. When balancing crankshafts prepared specifically for competitions, the computer displays the location where the shaft area weight needs to be extended.

Balancing is an essential crankshaft machining condition. Proper balancing improves ICE specifications:

  • engine power is increased by 10–15 %;
  • engine operates well in every mode and at idle speed;
  • fuel consumption decreases by 5–10 %;
  • vibration is eliminated.
Balancing is the final crankshaft machining operation after which no material may be further removed from the shaft body!
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