How Your Milking Machine Works

New Zealand Journal of Agriculture, Volume 63, Issue 5, 15 November 1941, Page 379

How Your Milking Machine Works

The PULSATOR

iiiiiitiiitniiutiiiiiu By . iiiniiiiiiuiiiiiiiiini

W. G. WHITTLESTON,

Animal Research Station, -* Wallaceville.

BEFORE discussing the individual types of milking machine pulsators it will be necessary to cover certain general ground. Hence, in the introductory portion of the article we will attempt to deal with— The Function of the Pulsator? The Action of the Milking Machine Teat Cups. ' r Methods of Testing the Pulsator.

Function of the Pulsator

The pulsator in a milking machine is a device which, by the production of an intermittent vacuum, causes an alternate compression - and release of the cow’s teats through the medium of a rubber liner or “inflation” in the teat cup.

Now, what is the object of this intermittent squeezing and releasing of the inflation by the pulsator? Firstly, let us look very briefly at the history of the pulsator. A steady vacuum was employed in the earliest vacuum-oper-ated milking machines, but they were not satisfactory. They were followed by machines in which the vacuum was caused to swing up and down (as in the so-called “vacuum break” machines). Some used metal' cups, and others used solid rubber cups which, under the in-

fluence of a varying vacuum in the inside, caused a certain amount of regular squeezing. These were not a great success. They did not milk quickly, and in many cases would not stay on 1 the cows during, a rapid flow of milk. Omitting some minor developments, the next important step was the development of the double-chambered cup in which a separate ■ intermittent pulsator vacuum was applied to the outside of the inflation and a steady vacuum applied inside—the modern cup with very few exceptions.

Modern cups employ two types of inflation in general—the soft and the moulded types. The former collapses generally when squeezing and so closes undei- the teat, as shown in Fig. 2. The action of a moulded inflation when collapsed is shown in Fig. 5. In

the latter case the inflation does not close beneath the teat, and so does not at any stage obstruct the flow of milk. The general principle of the modern pulsator is as follows: —Figs. 1 and 2 show • cross-sections of a typical “Thule” type cup. In Fig. 1 the rubber liner or “inflation” is shown in a relaxed position, which applies when the vacuum is the same on either side. Now the inner chamber A is connected by the nipple B to the milk pipe via the claw, while the outer chamber. C is connected by the nipple D to the pulsator valve. The latter connects the pulsator chamber of the cup C alternately to the vacuum (in the milk pipe in a single pipe machine, in the vacuum pipe in a double pipe machine) and to the air. In order that the relation of the squeeze and release phases, as we shall call them, may be clear, a pulsator valve is illustrated diagramatically in Figs. 3 and 4. In Fig.; 3 the valve is shown connecting the pulsator claw 'lead A to the vacuum lead B. In this position the inflation is at the position shown in Fig. 1 if the machine is of the orthodox type. Fig. 4 shows the pulsator valve connecting the claw pulsator

lead A to 1 the air. The vacuum lead in this position is simply blocked by the pulsator slide valve C. This position corresponds to the position of the inflation shown in Fig. 2.. In this position the atmospheric pressure acting on the outside of the inflation squeezes its walls together (the inside being partially evacuated) and so causes it to collapse.

How Does the Teat Cup Milk?

Sometimes we hear it said that a milking machine “milks on the squeeze.” By this is meant, of course, that the milk flows from the teat when the inflation is squeezing and the flow stops when the inflation releases.' In order to solve this problem the writer constructed cups with glass walls and fitted with inflations which had small celluloid windows let into them. As a matter of interest a photograph showing details of one such cup is given as Fig. 6. The following notes sum up many observations. In most cases milk flows within a few seconds after the cups are put on the cow, and is continuous for the first two or three minutes. There is no

visible evidence of a rise or fall in rate of flow with the action of the pulsator. There is certainly a slight choking of the milk during the squeeze with a slack soft inflation, but with a well-tightened inflation the milk seems to “get away” quite effectively during the squeeze phase. Towards the end of the milking the flow becomes erratic and shows little regularity, while when the machine is “stripping” in some cases there is a tendency for what little milk there is coming out to come on the squeeze. It is interesting to observe that if the pulsator is shut off the milk at first seems to flow normally in continuous streams, but it soon ceases. However, as soon as the pulsator is turned on the flow is resumed.

Theory of Milking Process

From the above, it would appear that the action of the machine is entirely different from hand-milking. In hand-milking the milk is actually extruded by blocking the upper portion of the teat and forcing the contents out through the bottom sphincter. In machine-milking . the milk is continuously drawn out by the action of the partial vacuum to which the teat is subjected. If we are to accept Dr. J. Hammond’s views (Vet. J. Vol. XVI, 17, 520) the following is briefly what happens. When the cow is about to be milked quite a fair portion of the total yield is contained in the milk cistern and the larger milk ducts. This is the milk drawn away by vacuum alone without the pulsator. However, the remainder is contained in the finer milk tubes (alveoli and ductules), and

cannot drain from these tubes without some mechanical action. This is provided by tissue in the udder which, under certain conditions, causes the fine milk tubes to contract, thus squeezing the milk out into the larger tubes, from which it runs freely to the milk cistern. Such action is brought about by the application of a suitable stimulation of the cow’s teats. If this theory is correct —and it seems to fit most known facts about the milking process— means that the sole function of the pulsator is to stimulate the teat and so to set up a nervous

reflex action which wffl cause certain . ~ ■ ~ , . tlssue m the udder to swell and force the milk down from the fine ; milk tubes of the udder.

Testing the Pulsator

It is important to ascertain just how a pulsator is functioning. This is most

readily done by the use of a recording vacuum gauge. A suitable instrument has been worked out at Wallaceville,

and the accompanying figures show the type of graph obtained when the vacuum at the pulsator nipples of the claw is recorded.

Firstly,, let us consider Fig. 7. In this diagram the vertical axis OY represents vacuum and OX represents time (the direction of motion of the tracing pencil is shown by the arrows). The graph represents the way in which the vacuum would vary in a perfect pulsator system operating at a 50:50 ratio, that is one-half of the total cycle is occupied by the squeeze phase, and one-half by the release.

In all subsequent references to pulsator ratio we shall mean the ratio duration of squeeze phase/duration of release phase. Sometimes the reciprocal ratio is used, but for convenience we shall always employ the above.

The curvature of the “vertical” portions of . the graph is due to the use of a recording instrument employing a simple arm with no correction for parallel motion. This is no disadvantage in the graph provided it is borne in mind when reading it, and it simplifies the instrument considerably.

Pulsator Ratio

Theoretically, the pulsator ratio is the ratio of the area BCDE to the area ABEF. In the ideal case this is equal to the ratio BC:AB, and for most practical purposes we may take the latter. In Figs. 8 (a, b, c) we see three different pulsator ratios: a = 40:60 b 30:70 , c = 60:40 The “snappiness” of the pulsator is indicated by the v sharpness of the angles at A, B, C, D, E, and F. In a poor pulsator these angles become

round curves, while a really snappy pulsator approximates to the form of the ideal graph. Needless to say, there are very few, if any, pulsators like this in practice. It must be borne in mind that the portions DC and FA represent the squeezing action. When this line slopes it means that the squeeze is sluggish and unstimulating. Similarly, the portions BE and GH represent the release phase when the vacuum, is applied again after the squeeze.

Mechanism of Pulsator

' Figs. 3 and 4 show the essentials of the pulsator valve. However, there are other types, and we shall endeavour to describe typical examples in order to show how they work and how to adjust and care for them. Pulsators may be classified into three main groups:—(1») Automatic Pulsators.— are essentially valve mechanisms operated by vacuum-driven motors. This class of pulsator was once much more popu-

lai- than it now is, but in view of the fact that many are still in use a description will be given of the main types. • ,

(2) Mechanically-driven Pulsators.In this type the pulsator valve is ' driven through suitable mechanism ■' from the motor which drives the vacuum pump. , (3) Miscellaneous.— The two chief types in this group, which is difficult to classify accurately, are the dia-phragm-controlled shuttle type of pulsator and the magnetic pulsator valve.

Automatic Pulsators

A general view of an automatic pulsator is shown in Fig. 9. It is essentially a small, single-acting vacuum engine which drives a double-acting pulsator valve. The driving force is generated in the cylinder (1), which contains a piston coupled to the connecting rod (2), which drives the flywheel (3) by the crank (4). When the piston is just past the bottom of its stroke the

small slide valve (5) driven by the flywheel shaft connects the cylinder to the vacuum line. Atmospheric pressure acting on the outside of the piston forces it into the cylinder, thus rotating the flywheel. When the piston reaches the top of its stroke the slide valve (5) opens the cylinder to the air, permitting the inertia of the flywheel to pull it back to the position where the cylinder is once more evacuated. The pulsator valve (6) is operated at one-half the speed of the slide valve (5) by means of gearing (7). This valve is shown diagramatically in figures 10a and 10b. In the positions

shown in Fig. 10a the valve is connecting lead (1) to the vacuum and lead (2) to the air. Fig. 10b shows the valve in the opposite position. This type of pulsator requires adequate lubrication. As the speed varies greatly with the load on the driving mechanism, it is important to use an oil which is not too viscous and to clear away regularly any old oil containing dust and dirt, which would impede the smooth working of the valves. All valves should be ground once in a while, although with care these pulsators will run for years without attention if well lubricated.

Another type of automatic pulsator is essentially a double-acting vacuum engine directly coupled to the pulsator slide valve. The device is shown in Figs. Ila and lib. As can be seen, it is in appearance rather complex, although the action is relatively simple. The two driving cylinders are shown at (1) and (2). These are connected to the vacuum via the piston valve system in the cylinder (3). The main pulsator valve (4) is of the double-acting variety, and on the same slide are drilled small auxiliary ports which operate the valve system. The mechanism shown at (5) is an ingenious device for halving the pulsator rate for the releaser.

The action of the device is as follows:—When the pulsator valve reaches the end of a stroke the auxiliary ports connect the valve piston to the vacuum in such a manner that the piston moves to connect the main cylinder on the opposite end to the vacuum, and the other main cylinder to the air. This causes the pulsator valve to move back to the opposite end of the stroke, where the process is repeated. The speed of the pistons is controlled by the two needle valves shown at (6) and (7).

The chief factors governing the speed of the pulsator once the needle valves have been set are:—(a) The vacuum: the higher the vacuum the faster the device operates; (b) the viscosity of the oil in the driving cylinders and in the valve cylinder. This also applies to most automatic type pulsators, and the same precautions are required for regular action.

Such devices need adequate cleaning and oiling from time to time, and the valves should be kept free from grit.

Pendulum Type Automatic Pulsator

The objection to both the previouslymentioned automatic pulsators is the fact that they vary their speeds with such factors as temperature, vacuum, etc. The pendulum pulsator is an automatic pulsator, operated , by two diaphragms and a slide valve mechanism. A pendulum is connected to the slide mechanism, and controls its speed of operation. This method effectively overcomes the problem of maintaining a steady speed with an automatic ■ pulsator. There are several types of automatic pulsator used only on bucket-type machines. As these have a very limited interest to New Zealand farmers, a description will not be given. The importance of automatic pulsators lies in the fact that they may be . used on milking machinery not driven by mechanical vacuum pumps. When a water-operated pump , placed some distance from the shed is used it is not

possible to employ a mechanicallyoperated pulsator unless a special motor is installed for this purpose only. It was this fact which made such pulsators so popular before the wide-

spread use of electrical power developed and when water-operated vacuum pumps were used in considerable numbers. V (To be continued.)