10.9.1 Basic Types of Nozzle System
The performance and characteristics of the cooling lubricant nozzle for the supply liquid lubricant have a major influence on the grinding result. A number of different nozzles have been developed in order to meet the requirements of various applications. Some of these nozzles are described in greater detail in Chapters 16 on surface grinding and Chapter 17 on cylindrical grinding. Generally, there are three ways to distinguish between types of nozzle systems [Heinzel 1999]:
• By function (flooding, not flooding)
• By focusing (free jet nozzle, point nozzle, swell nozzle, spray nozzle)
• By nozzle geometry (squeezed pipe, needle nozzle, shoe nozzle)
The primary task of all nozzles is the distribution of lubricants to the active zone. The nozzle carries out this task through focusing and directing the lubricant jet as well as accelerating the liquid. Investigations show a positive effect on the cooling performance by focusing the lubricant jet, associated with minimizing the turbulence of the flow by a sharp-edged exit of the nozzle or an extended parallel outlet of the nozzle. Additionally a minimum flow velocity must be generated, that is, by reducing the cross-sectional area at the nozzle outlet for wetting the wheel surface with cooling lubricant. The main obstacle to overcome for wetting the entire grinding wheel surface prior to its entry into the contact zone is a rotating air cushion around the grinding wheel [Marinescu et al. 2004]. Due to friction between the rough wheel surface and its surrounding atmosphere, a rotating air cushion is generated. The rotating air stream causes a permanent air flow away from the grinding wheel especially at high cutting speeds preventing grinding fluid from reaching the contact zone [Tawakoli 1990, Treffert 1995, Brucher 1996, Heinzel 1999, Beck 2001].
In order to breach the rotating air cushion by the cooling lubricant itself, a significant amount of kinetic energy has to be spent. The primary cooling lubricant nozzle can be used applying a higher flow rate and a higher flow velocity, which consumes more lubricant and significantly reduces the overall economic efficiency of the whole supply system. Another solution is to use a second nozzle in a radial direction to the wheel in front of the inlet of the actual cooling lubricant nozzle. Further improvement can be achieved by employing one nozzle for the peripheral surface and two additional nozzles for the side faces of the grinding wheel. In terms of an optimized design of the entire supply system, it is recommended to use a close-fitting housing for the grinding wheel.
Air guide plates closely aligned to the wheel surface are widely used to deflect the rotating air cushion away from the grinding wheel. Precise alignment and readjustment to a changing wheel profile or diameter are required [Tawakoli 1990, Brucher 1996, Heinzel 1999].