Friday, June 24, 2011

Connecting-rod aligner

Fig 7.25
This is a fixture that consists of a vertical faceplate, with a horizontal mandrel. The big end of the connecting rod is mounted on the mandrel and the rod alignment is checked against the faceplate.
A specially-shaped jig or V-block is used — the V of the jig is shaped to sit on the piston pin. Small dowels that extend from the jig are checked against the faceplate to determine rod alignment (Figure 7.25).
Checking alignment
Install the piston pin in the connecting rod, mount the rod on the mandrel of the aligner, and place the V of the jig over the piston pin.
Slide the rod along the mandrel until the dowels of the jig just touch the surface of the faceplate. If the dowels do not touch the plate, then the rod is misaligned.
The rod should be checked for bend and then for twist. To do this, the jig is mounted in two different positions on the piston pin:
with the dowels vertical. This will check for bend in the rod. If the rod is straight, both vertical dowels will be in contact with the faceplate (Figure 7.25(a)). If only one dowel contacts the faceplate, then the rod is bent.
with the dowels horizontal. This will check for twist in the rod. If the rod is true, and not twisted, both horizontal dowels will be in contact with the faceplate (Figure 7.25(b)). If only one dowel contacts the faceplate, then the rod is twisted.

·        The allowable misalignment, for both bend and twist is around 0.10mm. This is checked with feeler gauges between the dowels and the faceplate.

Straightening connecting rods
Connecting rods that are slightly misaligned can be straightened in a special jig designed for this purpose. A press or bending bar can also be used.
When straightening a connecting rod, it should be bent a little beyond the point where it is straight and then bent back to obtain correct alignment. This relieves the stress in the rod and it is less likely to tend to take up a permanent set and, for this reason, doubtful or badly misaligned rods should be replaced.

Analyzing piston, ring and cylinder problems
Engine noises, loss of engine power and exhaust smoke are indications that there could be problems with pistons and cylinders. While these could be due to wear in a vehicle with high kilometres, excessive wear or damage needs to be investigated.
The cause of a fault must be located and corrected so that the problem does not occur again. For damaged pistons and cylinders, do not just blame the part that failed, but consider the following as possible causes:
1. Operating conditions of the vehicle
2. Lubrication
3. Cooling
4. Temperature
5. Connecting-rod alignment
6. Other possible cause.
Following are descriptions of a number of piston faults that are not the result of normal wear. These are shown as separate faults, but some are a more advanced stage of one of the other faults, The cylinders in which the pistons operate will also be damaged, but the piston will show more damage because it is made of softer material.
When diagnosing a problem, consider whether it is related to the whole engine, or whether it is confined to only one cylinder. Also, consider whether the problem is related to the piston, the piston rings, or the cylinder, although one usually affects the others.

Scuffed pistons
Fig 7.26
Scuffing is a light abrasion on the thrust sides of the piston (Figure 7.26). It is caused by heat. It occurs when two surfaces are rubbing together and the temperature rises until melting point is reached. Small particles of metal then weld together, leaving small deposits on one surface and small holes in the other.
When scuffing starts, the damage can spread, being aggravated by the rough surfaces until an area of the piston is affected as shown.
The following should be checked for possible causes of scuffing or other piston failure:
I. The water-jackets for restrictions or deposits that could cause hot spots on the cylinder walls or distortion of the cylinder.
2. The lubrication system for correct pressure and type of oil.
3. The piston for size and clearance.
4. The connecting rod for alignment. 

See scored pistons>>>>>>>>>>>>>>>>>>

1 comment:

  1. Superb post to precisely describe the internal working of the engine especially the pistons, ring and cylinders.