9. Procedure
9.1 Charge the sample so as to reduce inclusion of air to a minimum. Soft greases may be poured into the cylinder or drawn up by vacuum; heavy samples must be hand packed. When filling the cylinder by vacuum, remove the capillary end cap and place the piston flush with the open end and then insert into the sample. Apply vacuum to the opposite end of the cylinder until the cylinder is fully charged with grease. This must be facilitated by tapping with a wooden block. Replace the capillary end cap and fill the upper end of the cylinder above the piston with hydraulic oil.
9.2 Fill the entire hydraulic system with hydraulic oil. Disconnect, invert and fill the gage and gage connections with oil. With the entire hydraulic system connected and completely filled with oil, adjust the temperature of the sample to the test temperature more or less 0.25°C (more or less 0.5°F) as determined by a thermocouple inserted in the capillary end cap. Operate the pump until oil flows from the gage connection on the viscometer before reconnecting the gage. With the entire viscometer assembled, circulate hydraulic oil with the return valve open until all trace of air is eliminated.
9.2.1 The time to attain test temperature varies with the bath. At -54°C (-65°F) the grease in an unstirred liquid bath should be ready to test in 2 h. Air baths can take as long as 8 h. An ASTM Thermometer 74F in the bath serves as a convenient secondary means of measuring the temperature at –54°C (-65°F). In an air bath the thermometer must be within 25.4 mm of the capillary.
9.3 With No. 1 capillary in place and the 40-tooth gear connected, operate the pump with the return valve closed until equilibrium pressure is obtained. Record the pressure. Change to the 64-tooth gear and again establish equilibrium. Record and relieve the pressure. Replace the No. 1 capillary with subsequent ones and repeat these operations until tests have been run with all capillaries at both flow rates. With some soft or hard greases, it cannot be practical to use all of the capillaries.
NOTE 5 - It may be necessary to refill the cylinder with fresh grease when all 16 determinations are to be made.
10. Calculation
10.1 Calculate apparent viscosity of the grease as follows:
where F is the shear stress, and S is the shear rate. Therefore:
where:
p = pressure dynes/cm2,
L = capillary length, cm,
P = observed gage pressure, psi (multiply by 68944 to convert to dynes per square centimetre),
R = radius of capillary used, cm, and
v/t = flow rate, cm3/s.
10.2 Calculations may be reduced to a minimum by preparing a table of 16 constants, one for each capillary and shear rate (Table 1). For example, viscosity with No. 1 capillary and the 40-tooth gear is given as follows:
where:
10.3 Also calculate the shear rates as follows:
Correct the flow rate to correspond to the observed pressure by reference to Fig. 3. Calculate 16 shear rates for the eight capillaries and two flow rates. This calculation need not be repeated for each run since it will remain constant until recalibration of the pump indicates a revision.
10.4 Plot a curve of apparent viscosity versus shear rate on log-log paper, as shown in Fig. 4.
NOTE 6 - Shear stresses also can be calculated by multiplying apparent viscosities by their corresponding shear rates. For solving various problems involving the steady flow of greases, shear stress-shear rate relationships may be plotted on appropriate charts. Instructions on the use of these charts are given in the article by Rein and McGahey, "Predicting Grease Flow in Large Pipes", NLGI Spokesman, April 1965.