Consider the following hypothetical scenario. The decision has finally been made: a centralized grease system will be installed in the wash area to lubricate the many greased bearings that require daily visits.
Unfortunately, the only location deemed suitable for the grease tote and pump is in the basement of the building. The piping will have to extend from the basement through a section of the building that is exposed to the direct blow of that cherished Nebraska winter cold. Additionally, the pipe run will be a couple hundred feet from the pump to the distribution block farthest from the grease tote.
So as the lubrication program manager (LPM), it is up to you to make sure the system design and product selection will accommo- date the anticipated low-temperature requirement. The bearings require grease with a high-viscosity base oil to accommodate the low speeds and high loads, a moderately high dropping point to accommodate the moderately high temperatures, and exceptional corrosion wash resistance to accommodate the profuse process water contact to which the bearings are exposed.
The central question is this: Is it possible to know whether the NLGI No. 1 grease with an ISO 680 base oil that is currently in use will flow through the pipes and metering valves effectively during the winter months without performing a trial run?
Fortunately, the answer is yes, it is possible to know without risking system failure or potential loss of production time. The property that must be identified is the “shear rate” (flow property), and the characteristic that must be determined is called “ventability.”
The intent of this article is to provide a brief overview of the concepts of shear rate and flow property, and to review the theory, operation and use of the device to measure these properties: the Lincoln Ventmeter.
When centralized grease lubrication was introduced in the mid-1930s, the manufacturers of these centralized systems needed to understand the properties and flow characteristic of grease. Grease is classified as a non-Newtonian thixotropic pseudo plastic (viscosity decreases as the shear rate increases) fluid. This means the viscosity is reduced as the shear rate increases. Figure 1 illustrates three types of viscosities.
The general indication of grease’s flow property by NLGI rating is not sufficient to predict flow performance across a temperature gradient. This information is necessary in designing and specifying a centralized grease lubrication delivery system.
For example, an NLGI No. 2 grease may behave like a NLGI No. 3 at 50°F. Another grease with NLGI No. 2 may behave like a NLGI No. 3 at 30°F. The uncertainty of the grease flow properties at different temperatures hinders system designers and end-users trying to determine the safe line size to ensure proper operation of an automatic lubrication system.
The operation of a single-line lubrication system is straightforward. When lubricant is needed, the controller opens an air solenoid to turn on the pump. The pump produces flow and builds up pressure in the line. When the pressure reaches 1,800 psig, the injectors distribute a predetermined amount of lubricant to a bearing. A pressure switch usually located farthest away from the pump senses when the pressure has reached +/-1,800 psig.
Once reached, the pressure switch sends a signal to the controller indicating that the system pressure was achieved. The controller then turns off the air solenoid valve and thus the air supply to the pump. When the air supply is turned off, a three-way valve is activated which directs the grease in the line directly back to the reservoir. Thus, the pressure in the system can be bled off. This is known as “venting” the grease.
Because a centralized lubrication system may require the grease to be pumped a long distance, if the grease is too stiff, then the pressure may not relieve (vent) back to the reservoir in time for the next lubrication cycle. For proper operation of the injectors in these systems, and to prevent the grease from being subjected to constant pressure, venting of the grease to a minimum pressure level is required.
Figure 2. Single-Line Lubrication System
By measuring the flow properties of grease, a lubricant designer or an application engineer or technician can select the pump and line size to ensure good performance of the centralized grease lubrication system, or select an appropriate lubricant for an existing system. The Ventmeter provides this information.
The apparatus provides a quantitative measure of the minimum shear stress to produce a flow. Its usefulness is most noted in three ways:
By definition, the minimum shear stress determines the ability of the grease to vent. Ventability is defined as a measure of residual pressure (psig) where a grease ceases to flow in a right cylindrical tube of coiled ¼-inch (6 mm) diameter tubing 25 feet (7.62 meters) in length.
The yield pressure of the grease is obtained in the following manner: The sample of grease is charged to 1,800 psig through a grease gun (Figure 3).
After a pressure of 1,800 psig is reached, a relief valve opens and the grease is discharged. The discharge of the grease reduces the pressure. The pressure left in the system is read 30 seconds after the relief valve is opened. Thirty seconds is enough time for the grease to stabilize. The gauge reading is the Ventmeter viscosity. The test is conducted normally at ambient, 30°F and 0°F. In this way, the yield pressure is obtained at various temperature conditions. If the gauge reading goes to zero within the 30 seconds, the grease has effectively no yield limit.
Tests are performed at progressively lower temperatures to establish a value when the grease ceases to flow. Such greases behave like oil, for practical purposes, and can be considered Newtonian. With lower temperatures or stiffer greases, the gauge reading will be some value other than zero.
The science behind the ventmeter was first presented in the 1965 issue, Volume 29, of the NGLI Spokesman. The results showed that the ventmeter could be reliable in determining a grease’s yield pressure and apparent viscosity by calculating the shear rate in conjunction with the shear stress.
The basis of the calculation is Poiseuille’s equation for apparent viscosity, as follows:
Consider this practical example (of calculation used in Table 1): Say a test sample was shown to have a ventmeter reading of 400 psig (27.5 bar). Consider the fact that the radius of the steel coil in the ventmeter is .125 inches (3 mm) and is 25 feet (300 inches or 6.72 meters) long.
The equation for yield pressure, in English units:
The grease manufacturer wants to know if the grease with a ventmeter reading of 400 psig can be used in a centralized automatic system using one-half inch (8 mm) of high-pressure hose with injectors that specify a vent pressure of 600 psig (41 bar) and a supply line length of 65 feet.
Using these parameters of ½-inch diameter (0.250 inch radius or 6 mm) tubing and 600 psig (41 bar) venting pressure of the injectors, the following is determined by solving the equation for shear stress in terms of length:
Because the supply line is 65 feet long, the grease would be suitable for use in this system.
Standardized tables can be generated for a particular style or type of injector and grease distribution system. The popular Lincoln injectors are available in various sizes. The supply line rating chart for the SL-1 and SL-11 style injectors, with venting pressure requirements of 600 psi, is noted in Table 1.
A grease with a Ventmeter reading above 600 psi would not be suitable for use with automatic lubrication systems.
There are additional tables for other sizes and styles of injectors.
The main purpose of a centralized lubrication system is to reliably deliver the right amount of grease at the right time. This usually means a small amount of grease applied frequently. The successful operation of the centralized lubrications can help machines operate at peak performance for a long time. Understanding grease shear properties and correctly matching system design with grease behavior (type) is a key step in appropriate system engineering. The Lincoln Ventmeter is a tool to determine any given grease’s suitability for use in a centralized lubrication system.
The NLGI Rating
The NLGI rating was developed by the National Lubrication and Grease Institute to provide an indication of the consistency of grease. Most manufacturers of grease include the NLGI rating with the product specifications. This static measure of stiffness provides a general indication of the grease’s resistance to penetration, and therefore displacement under load. The NLGI measurement method is a worked penetration test, meaning it measures resistance to penetration under repeated load/unload cycles. While greases with higher NLGI ratings are typically more resistant to flow, the NLGI method does not directly measure pumpability in actual applications. It may not accurately correlate.