LO 5-4 Define concentricity and explain methods to measure it

LO 5-04: Define concentricity and explain methods to measure it.

Measuring Concentricity

Measuring concentricity of an object involves determining how closely a circular object is centered around its axis. In other words, measurement of an object’s concentricity determines how circular it is or whether it has runout and how much runout is present. An example of a concentricity measurement would be of a rotating shaft (such as a crank- or camshaft) bearing journal surface FIGURE 28. The shaft is first supported at each end on narrow blocks called vee blocks , where it is slowly turned. Next, a dial-type indicator with a roller-type tip is placed against the journal surface around the shaft’s centerline. A movable plunger on the dial indicator transmits any lateral movement of the shaft to a rotating gauge, enabling measurement of how much movement takes place. When there is no difference between the axis of the shaft and its outer surface, the shaft is said to be concentric. Runout of the shaft, which is where the pointer of the dial indicator moves in or out while rolling over the shaft surface, means the shaft is not concentric. Camshaft lobe lift is measured with continuous-type dial indicators and is expected to be nonconcentric.

FIGURE 05-28: Measuring camshaft lobe lift. The lobe will be nonconcentric with the centerline axis of the shaft.

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LO 05-4 Animation: Measuring Camshaft Lift (FULL SCREEN)

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LO 5-04: Define concentricity and explain methods to measure it

Dial Bore Gauges

Dial bore gauges are used for precision measurement of inside diameters of cylinder bores. These gauges use a dial indicator attached to a long handle to quickly inspect a cylinder, either new or worn, for cylinder concentricity, wear resulting in taper, or barrel-shaped cylinder dimensions. When the dial indicator is calibrated with a standard dimension ring gauge having the same specific diameter of a cylinder, the gauge can quickly determine whether the cylinder is bored to specifications. An alternative method is to zero the bore gauge to a dimension measured by a micrometer. Plunger extensions of various lengths can be interchanged on a single dial bore gauge to inspect cylinders of a wide range of diameters.

When placed inside a cylinder bore, the smallest reading on the dial gauge is observed at the narrowest point in the cylinder when the gauge is square, or at 90 degrees perpendicular to the cylinder bore. This position is found by rocking the dial bore gauge lever back and forth inside the cylinder FIGURE 29. The dial indicator pointer will move to its smallest number and then change direction on either side of 90 degrees, perpendicular to the cylinder where the measurement is taken. To understand how a dial bore gauge is used to measure cylinder dimensions, follow the outline describing its use in SKILL DRILL 5-2.

FIGURE 05-29: The narrowest dimension of a cylinder bore will be at 90 degrees or perpendicular to the axis of the cylinder bore.

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SKILL DRILL 05-2  Using a Dial Bore Gauge

To measure the diameter of a cylinder using a dial bore gauge, use the following steps as a general guideline: STEP 1 Using a ring gauge (made to the same precise size of the cylinder bore diameter) or an outside micrometer, zero the dial gauge inside the spindle-anvil of the micrometer or ring gauge. To do this, turn the outer ring or bezel of the dial gauge to align the zero graduation with the stable pointer position. STEP 2 After zeroing the gauge, move it to the cylinder and tilt the gauge to place it inside the cylinder bore. STEP 3 Rock the lever of the bore gauge back and forth, sweeping the gauge through a perpendicular position inside the cylinder. When the handle is at exactly 90 degrees to the cylinder, the dial gauge plunger will record the narrowest dimension of the cylinder due to the plunger’s orientation with the cylinder walls.

STEP 4
When rocking the gauge, observe and record how far the dial plunger is either pushed into the gauge (a negative reading relative to the zero position) or how far the plunger moves past the zero position (a positive reading).

STEP 5
Take measurements in two places, 90 degrees apart, at the top, middle, and bottom of the cylinder where the piston has traveled.

STEP 6
Include separate measurements near the top of the cylinder above the uppermost area of piston ring travel (but not on the cylinder ridge of a worn engine). Measure the area near the bottom of the cylinder, below the lowermost point of ring travel.

STEP 7
Record all observations and analyze the results to determine the amount of taper, belled shape, and barrel wear of the cylinder concentricity. Compare measurements with specifications and make service recommendations.

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