The crankshaft has doing heavy loads, torsion, bending and mechanical wear during the working. The torsion moment that developments on the crankshaft, has been transmitted to transmission and used to drive others mechanism of engine, so the crankshaft is an essential part of the power transmission system. You will have known what crankshaft machining means, crankshaft diagnosis and methods of crankshaft machining when you read this article to the end.
Crankshafts have a relatively low rigidity and they are easily deform under the influence of radial and axial loads; therefore, during processing (especially finishing), it is necessary to take measures to prevent deformation.
There are some stages of crankshaft machining: roughing, finishing and fit out (polishing, grinding).
Roughing and finishing of the main journals and the ends of large crankshafts perform on lathes. The crankshaft journals located close to the end are processed firstly, then the shaft with these journals is installed in the lunettes, after that other journals are processed. Such subsequence allows eliminating the deformation of the shaft in the centers. At the same time, we need to process all surfaces whose axes of rotation coincide with the axis of the main necks: the end surfaces, contours and bevels of the cheeks. Journals of small crankshafts are machined on multi-cutter lathes with double-sided or central drive.
Install the shaft with machined middle or extreme journals in a rotating bezel or special chucks. At that time, part or all of the main journals free from clamping and the end surfaces of the cheeks are machined.
Three prismatic incisors process each crankshaft journal or pair of cheeks. Processing the end surfaces of the cheeks, fillets and adjacent parts of the journal does two cutters of the front caliper and the third cutter of the rear caliper processes the middle part of the journal.
The cutters work as shaped cutters with a radial feed. Because the cutters are located on both sides of the processed neck, the deformation of the shaft is reduced.
Crankshaft`s roughing and finishing are carried out on the same machines. But, they have some differences in the setting (the size of the seats in chucks and lunettes, the size and shape of the cutters).
Processing of the connecting rod journals and the surfaces of the journals, the axes of which are not aligned with the axis of the main journals is very specific operation. Such operation does on the machines with a rotating caliper. You can see it on the scheme.
Processing of crankshaft`s connecting rod journals and journal surfaces on the turning machine
- Do the fix of crankshaft with main journals in the prism of the struts 4.
- Turn the crankshaft and displace the body along the guides in the transverse direction to alignment of the axis of the neck to be machined with the axis of rotation of the cutters.
- Two calipers move on prismatic guides mounted on a ring , which rotates in the body with the help of a worm gear pair.
- Saddle feed carries from an electric motor with a gearbox through lead screws.
- The installation of the shaft is checked using a clamp. This clamp is used to change the distance from the rotating ring to the center of the neck, which is numbered on the cheek.
- The cheeks are trimmed simultaneously with two incisors moving towards each other. The neck is pierced with cutters set to size when the body moves along the guides 2 along the axis of the neck.
The turning of the connecting rod journals of small shafts with a large output is carried out on machines with a double-sided drive; in this case, the liners in the chucks for installing the shaft are displaced by the value of the crank radius. From one installation, the necks located on the other axis are processed. The angular position of the shaft in the chuck during processing of the extreme connecting rod journals is fixed along the base platform or the risk on the cheek, and when processing the remaining journals – along the processed extreme connecting rod journal.
Machining is carried out with two cutters from the front carriage and one cutter from the rear carriage, in the same way as when processing the main journals. Rectangular crankshaft cheeks are machined on vertical milling or longitudinal milling machines with end mills.
Round cheeks are processed on lathes, mainly with a double-sided drive, similar to the processing of connecting rod journals. Processing is usually carried out with straight cutters with a longitudinal feed. Oval-shaped cheeks are processed either in parts in the same ways as round cheeks, or on multi-support turning-copying machines, structurally similar to machines for processing camshafts, the scheme of which is shown in scheme.
The crankshaft journals are finished after finishing. The necks of very large shafts are finished on lathes at the same time as finishing. This work is carried out manually by highly qualified workers. The shape of the surface of the necks is checked for paint using standard steel liners, and the dimensions and mutual position of the necks are checked with micrometers and indicators.
Currently, the finishing of the necks of the crankshafts of diesel locomotive and marine engines is carried out in the same way as for small crankshafts, on special grinding machines. Grinding of the main journals is carried out with the installation of the shaft in the centers and rests. When grinding large shafts, the installation of the rests is checked by monitoring the change in the distance between the cheeks with an indicator device.
If the shaft bends during rotation, the distance between the cheeks changes. In this case, a change in the distance between the cheeks is not more than 0.01-0.02 mm. By adjusting the rests, the correct position of the shaft axis is achieved, and in this position the journals are ground. Grinding of the connecting rod journals is carried out on grinding machines with a double-sided drive (see Figure).
The crankshaft, as well as when turning the connecting rod journals, is installed with the end main journals in cartridges with eccentrically located liners. At the end of the shaft, a dividing disc with grooves is fixed, which is held in the required position by a retainer.
With unclamped indexing disc and chucks, the shaft is installed so that the crankpin journals to be ground have minimal runout. Then the cartridges and the dividing disk are fixed on the shaft, and the shaft is firmly fixed with clamps , after which the necks are sequentially ground, the axes of which coincide with the axis of rotation of the machine spindles. Grinding is carried out with gradual pressing of the lunettes installed under the necks to be ground.
After grinding a pair of necks, the shaft is turned until the axes of the next pair of necks are aligned with the axis of rotation of the machine spindles, the retainer is inserted into the groove of the indexing disc and then the next pair of necks is ground. When grinding the necks on specialized machines, the dimensions are controlled by an indicator three-pin bracket.
After grinding, the necks of the shafts are polished or superfinished. The scheme of the machine for superfinishing of the crankshaft is shown in Figure . Abrasive bars of the heads for superfinishing are pressed against the journals of the crankshaft rotating in the centers of the machine.
The heads are fixed on the crankshafts 2 rotating synchronously with the workpiece, as a result of which constant contact of the bars with the journals is ensured. The eccentric pin 4 rotating from the electric motor imparts to the bars reciprocating movements along the neck. Work is carried out with abundant cooling of the part with kerosene.
Scheme of operation of the machine for superfinishing of the crankshaft journals.
The journals are polished on similar machines, but instead of abrasive bars, they have presses that cover the shaft neck and press fine-grained abrasive paper to the treated surface.
To supply grease to the bearings, holes are made in the journals and cheeks of the crankshafts of the parts. These holes have a small diameter (6-10 mm) and a great depth. In small-scale production, lubrication holes are machined using jigs on radial drilling machines. In large-scale and mass production, special machines are used for this, often multi-spindle, working with private drill retracts for chip removal, or automatic lines.
The retraction and advance of the drills is carried out automatically after drilling 4-5 mm. To prevent breakage of drills, deep hole drilling machines are often equipped with devices that retract the drills when a torque occurs.