A good rack allows all the balls to touch when pushed forward and the rack is lifted from the table. Even with a good rack, if the table cloth is uneven from dings, balls can separate. If the rack is properly made, the balls should cluster together when pushed together. A poorly designed rack will leave gaps between the balls. (The balls could also separate because of unequal diameters.)
With a good rack, even a medium speed break shot will separate the balls well. A bad rack with gaps in between balls will absorb energy and lead to clumped and clustered balls, even with a strong break.
On tables where a decent rack of balls is impossible to set up, ensure that the top three balls of the rack are touching as best as can be done. If a gap does appear, regardless of your best efforts, inform your opponent and invite him to set up the rack. (Almost all opponents are very tolerant when they realize that a faulty rack is a table condition.)
Sometimes it is loose chalk dust or simply air dust that has settled down onto the table and then mounded or clumped together because of balls traveling back and forth over the area. And don't forget that on a solid stroke with a properly chalked tip, the chalk flies off in a shotgun scatter pattern. You can partially offset this affect and get the balls to group better by brushing or slapping the cloth around the racking area. This vigorous action will, at the least, make the area more level and even. There will be less chance of the balls sitting on a small ridge and rolling away.
Several companies manufacture special tight tolerance machined racks. The cost factor will be a deciding point for you.
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