Certain presumptions are made in a theoretical Roberval balance. In order for such a balance to appear level in its natural state and be able to balance theoretical masses, the following must be true:

• All six pivot points must move without producing friction (since Roberval balances often actually require twice this number, a total of 12 pivot points would need to be friction free)
• The lengths of the arms (left and right of the fulcrum) must be exactly equal unless

• The weights of the arms themselves are unequal, or
• The weights of the vertical columns and/or pans is unequal

• The vertical distance between each vertical set of pivot points must be exactly the same
• In order to be balanced front-to-back the balance must either have two sets of two arms located around a central fulcrum or must have two fulcra supporting a single set of arms
• The weight of the arms on each side of the fulcrum must be equal (unless see above)
• The arms must be rigid and inflexible
• Gravitational force or rotational G force must be acting uniformly on the balance
• If the weight of the pan above either vertical column is itself greater than zero and any weight placed on that pan is off-center then that pan's tendency to tilt will cause the balance to exist in a state of tension at the pivot points below that pan. This tension will manifest as an increase in static friction.
• The longer the arms generally, the more sensitive the balance, though longer arms usually entail greater arm weight, which tends to decrease sensitivity
• Heavier pans and vertical columns also tend to decrease sensitivity
• Sensitivity lost by increases in either arm weight or pan/column weight can be counteracted only through decreased static friction in the pivot points
• The upper pivot point of the central supporting column is prevented from moving left–right and front–back by the fulcrum itself; it is prevented from moving up–down by gravitational pull. The lower pivot point of this column must be held in place so that it cannot sway left–right and, to a lesser extent, front–back as the arms move, but it experiences no up–down movement forces — in this arrangement, the entire pivot process takes place on the upper central pivot point, which acts as the single fulcrum for the entire balance; it is possible to reverse this, so that the lower pivot point acts as the fulcrum and the upper is only held in place so that it cannot sway left–right or front/back.
• Roberval balances are frequently depicted with the "pan" as a plate or peg protruding from the center of each vertical column— this is so that the balance can have a center of gravity that is in the actual center of the parallelogram and so that adding weights to these plates does not change that center of gravity. This produces the somewhat odd result that a correctly balanced Roberval balance, unlike a beam balance, can be "balanced" in any arm position: so long as the masses of the objects on both sides are equal or the pans are empty, it will balance with the right arm up and the left arm down, as well as the left up and the right down, as well as any position in between, and all of these positions will be "correctly balanced".

• As a corollary, because no actual two masses can have exactly the same weight, a highly precise Roberval balance measuring two such imprecise masses should always tip either completely to the left or completely to the right— it does not measure degree of difference, it indicates the existence of difference. These effects must be distinguished from the feedback loops and the friction of the pivot points mentioned above, as those are undesirable effects caused by design weaknesses or flaws.

• The correct method of using a precise but real Roberval balance, then, is to place one of masses (either the known or the unknown) on one plate/pan and then add only enough of the other mass to the other pan until the balance just barely tips completely in the direction of the second added mass. If the arms finalize in a horizontal position, this only indicates friction in the pivot points somewhere. A well-made and precise Roberval balance with a centralized center of gravity never actually "balances".

The Roberval balance is arguably less accurate and more difficult to manufacture than a beam balance with suspended plates. The beam balance, however, has the significant disadvantage of requiring suspensory strings, chains, or rods. For over three hundred years the Roberval balance has instead been popular for applications requiring convenience and only moderate accuracy, notably in retail trade.

Well known manufacturers of Roberval balances include W & T Avery Ltd. and George Salter & Co. Ltd. in the United Kingdom and Trayvou in France. Henry Troemner, who designed scales for the United States Department of Treasury, was the first American to use the design.^[1]

• J. T. Graham, Scales and Balances, Shire Publications, Aylesbury (1981) ISBN 0-85263-547-8
• Bruno Kisch, Scales and Weights. A Historical Outline, Yale University Press, New Haven (1966) ISBN 0-300-00630-6

• The Roberval Balance
• Physics Demonstration showing surprising paradox of simple Roberval Balance design