Cluster components – getting it right
Cluster weight
The weight of commercially available clusters varies from about 1.6 to 3.5kg. The optimum weight for the commonly used liners
in New Zealand tends to be in the range of 2.2 – 2.6 kg. The main benefit of increasing cluster weight is to reduce the amount of
milk (‘strippings’) left in the udder when cups are removed. However there are disadvantages to increasing the weight of the
cluster including: increased milker fatigue, slipping and falling of cups and teat damage. The choice of an ‘optimum’ weight
usually involves a compromise based on the type of liner used and the preferred vacuum setting.
Cluster weight in relation to vacuum level
The average vacuum level in the claw during the peak flow period of milking should be within the recommended range of 34-42
kPa. Note that cluster vacuum is not the same as the working vacuum (as per the vacuum gauge) and requires specialist
equipment is needed to measure it.
Clusters that do not hang evenly on the udder
Uneven weight distribution between the four quarters of an udder is one of the most common causes of incomplete milking,
uneven milk-out, and liner slips. The milking unit should hang squarely on the udder so that cluster weight is evenly distributed
across the udder’s four quarters. This rarely occurs in practice. Usually, the main culprit is the long milk tube, although udder
conformation also has a major influence e.g. 3 titters. See “Aligning clusters for efficient milking” (below) for solutions to poor
cluster alignment.
Mismatch between the claw inlet and the short milk tube (SMT)
Flattened or kinked SMTs indicate common errors in matching liners and claws. In New Zealand, liner SMT bores vary from 814mm with a corresponding variation in claw nipple size. The same liner model is often produced with two different SMT bores
to accommodate different claws. Seek expert help to ensure the liner SMT and claw are compatible or restriction to milk flow
may occur.
Desirable claw features
Scientific studies suggest that claws must have an effective volume of at least 150ml to provide stable vacuum below the teat,
minimise cross-flow and contamination between quarters and minimise liner slip. Other important characteristics of claws
include:
appropriate weight;
unrestricted inlet nipples and claw outlet to allow free flow of milk;
good visibility of milk flowing into the claw bowl from each quarter;
ruggedness in a tough environment;
be relatively easy to open and maintain;
the elusive, but important, quality of good ‘personal feel and handling characteristics’;
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visibility of milk in the claw bowl to help detect problems such as blood in the milk and to provide an indication of the
end of milking for individual cows (unless using MaxT see Chapter 4: The Milking Process. Managing Slow Milking
Cows).
Cluster air admission
The majority of clusters have a small air vent that admits air during milking (one brand allows air admission on the liner instead),
preferably in the range of 10-12 L/min per cluster. This vent helps to remove milk from clusters. Excessive air admission (greater
than 12 L/min) tends to reduce claw vacuum, cause more milk frothing and increase free fatty acids in milk due to damage to
milk fat globules (lipolysis).
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