Friction between the teat and teatcup liner during milking

Abstract

Four studies are described of the role of friction in maintaining the teatcup stable on the teat. Measurements of the coefficient of friction between teats and pieces of liners, in which most values for the coefficient fell between 0-5 and 1-0, indicated that friction between skin and rubber-like materials was consistent with the general frictional behaviour of elastic solids. Studies during milking showed that the sudden restriction of milk flow that normally occurs near the end of milking is accompanied by a marked fall in the frictional force between the teat and barrel of the open liner. During the period of peak milk flow-rate, the major source of friction maintaining the teatcup stable on the teat is the large area of contact between the teat and liner barrel. The frictional force is derived from the pressure difference across the teat wall which presses the teat against the comparatively rigid liner. Frictional force between the teat and barrel increases after the start of milking because the coefficient of friction rises as one surface gradually moulds to the other. In addition, the total frictional force increases because of the increasing area of contact whenever the teat moves deeper into the liner, until the end of the peak flow-rate period. When this period ends, friction between the teat and open barrel is reduced suddenly because the teat sinus pressure falls. After this stage, the main source of friction appears to be derived from the force between the teat and mouthpiece lip.When a teatcup assembly is in place on a teat during milking, there are opposing forces acting both to remove the liner from the teat and also to cause the teat to move farther into the liner. The force tending to separate the teat and liner is due to the weight of the teatcup assembly. The opposing force tending to thrust the teat deeper into the liner is proportional to the vacuum level in the liner and the crosssectional area of the teat exposed to the vacuum. Generally, this force is greater than the force due to weight when the liner is open, but less when it is closed. Clearly, a source of additional force is required to stop the teatcup moving up or down the teat. Recently, it was reported that frictional properties of liners appeared to have an important and progressively increasing influence on the depth of penetration of the teat in the liner during the course of a milking (Mein et al. 1970). Four subsequent studies of the role of friction in maintaining the teatcup stable on the teat are described in this paper.

Effects of vacuum level and pulsation rate on milk ejection and milk flow traits in Tunisian dairy camels (Camelus dromedarius)

“…Nevertheless, a too low vacuum could be counterproductive and could result in a longer milking time and a higher frequency of the teat cups slipping and falling off (Spencer and Rogers 1991) that could be deleterious for udder health. On the contrary, higher vacuum shortened milking process by increasing milk flow in cluster and tubes but can induce injury (petechial hemorrhages, teat sphincter eversion…) and even reduce milk flow at the end of milking because of teat tissue edema (Hamann and Mein 1990) and liner climbing that can also increase stripping milk by closing the cricoïd ring (Mein et al 1973). …”

Factors affecting mouthpiece chamber vacuum in machine milking

“…Forces generated by the liner vacuum act to pull the teat deeper into the liner, and gravity acts to pull the teatcup off the teat. To maintain a stable position on the teat, the teatcup depends on frictional force between the teat and the liner wall (Mein et al 1973).…”

“…This results in a pressure gradient that presses the teat against the liner. Because of increased friction, it requires a greater force to pull the teatcup off the teat when the MPC vacuum is high compared with lower MPC vacuum, and the risk of liner slips during peak flow rate is reduced (Mein et al 1973).…”

Teat shape associated with teat cup crawl and production in Friesian cows

“…Recently Mein et al (1970) reported that frictional properties of liners appeared to have an important and progressively increasing influence on the depth of penetration of the teat in the liner during the course of a milking. Mein et al (1973) further reported that during the period of peak milk flowrate, the major source of friction maintaining the teat cup stable on the teat is the large area of contact between the teat and liner barrel.…”