Understanding Required Balancing Tolerances

The following drawing defines the required balancing tolerance according to  ISO 1940/1.standards:

NOTE: The table in my previous blog defines the required balancing quality G according to each rotor type.

The maximum service speed is reported on the horizontal x axis , while the acceptable specific unbalance (acceptable unbalance per unit of mass or acceptable residual mass eccentricity ) is reported on the vertical y axis.

The following formula can be used instead of the previous diagram:

Where: Et [] = total acceptable mass eccentricity
N [RPM] = Maximum service rotor speed
G [mm/s] = Balancing quality or grade
Total residual accepted unbalance: U [gr∙mm] = Et∙M
where: M [kg] = Rotor mass
Total residual admitted unbalance in grams is  where R [mm] is the compensation radius.

This is all very technical stuff and most likely, only of interest to a very small number of people who enjoy the complexities of balancing as much as myself!

If you would rather leave the formulae to the experts then why not have a chat with a balancing machine expert at Cemb Hofmann UK.

Call us today on 0161 872 3123 and we will be delighted to assist you with whatever area of unbalance you are experiencing with your rotating parts.

A Guide to Balance Qualilty Grades

The chart below is designed to give an overview of the balance quality grades for various groups of representative rigid rotors.

Note:  Some groups of rotors, not included in the official ISO table, are added and reported in Italic type form.

Balancing quality grade is show in G mm/s

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0,4    Gyroscopes
         Spindles, discs and armatures of precision grinders
         Textile fuses

1,0    Small electric armatures with special requirements
         Tape recorder and phonograph (gramophone) drives, cine projectors
         Grinding machine drives
         Turbines  and Compressors with special requirements

2,5    Gas and steam turbines, including marine main turbines  (merchant service)
         Turbine driven pumps
         Rigid turbo generator rotors
         Turbo compressors
         High speed compressors and aeronautic compressors
         Medium and large electric armatures with special requeriments
         High quality household electric armatures, dentist drills and textile components
         Small electric armatures not qualifying for one or both of the conditions specified
         for small electric armatures of balancing quality grade G 6,3
         Machine tool drive
         Air conditioning fans for hospitals and concert halls
         High speed gears(over 1000 RPM) of marine turbines .
         Computer memory drums and discs

6,3    Small electric armatures, often mass produced, in vibration insensitive applications, and/or
          with vibration isolating mountings.
          Medium and large electric armatures (of electric motors having at least 80 mm shaft height )
          without special requirements
          Machine tool and general machinery parts
          Parts of process plant machines, Centrifuge drums, decanters, washers
          Hydraulic machine rotors
          Fly wheels, Fans; Pump impellers
          Marine main tuebine gears (merchant service )
          Paper machinery rolls; print rolls
          Assembled aircraft gas turbine rotors
          Individual components of engines under special requirements

16      Drive shafts (propeller shafts, cardan shafts) with special requirements
          Parts of agricultural machinery, parts of crushing machines
          Individual components of engines (gasoline or diesel) for cars ,trucks and locomotives
          Crankshaft / drives of engines with six or more cylinders under special requirements
          Low speed separators
          Light boat impellers)
          Motor bicycle and car wheels
          Normal transmission pulley
          Wood machine tools

40      Car wheels, wheel rims, wheel sets and drive shafts
          Crankshaft / drives of elastically mounted fast four cycle engines (gasoline or diesel ) with
          six or more cylinders (pistons speed greater than 9 m/s)
          Crankshaft /drives of engines of cars , trucks and locomotives

We hope that you will find this information useful - please get in touch with the specialist balancing team at Cemb Hofmann UK on 0161 872 3123 should you have any queries regarding this table or any other aspect of balancing.

Please visit our site: www.cembhofmann.co.uk for more information about balancing machines and sub contract balancing services.

Balancing Tolerances

The balancing of a rotating body has different goals:

1) Reduced load on the bearings (low centrifugal forces)
2) Long bearings life
3) Acceptable vibration levels (a good vibration level does not create any problems to the comfort or to component life.

From previous point 3 , it is clear that the optimum value for the residual unbalance can be evaluated in an experimental mode, by considering that:

a) The inertia force generated by the unbalance can be calculated using the formula reported on paragrath 1.15

b) On service vibrations, levels can be easily measured with a simple vibrometer.
For each application an acceptable value for the admitted residual unbalance (which grants good performances ) can be defined.

ISO 1940 standards gives a rule in order to calculate an acceptable residual unbalance, having following features:

1) Gross unbalance deficiencies are avoided,
2) Useless and excessive balancing works are avoided

Where:    E = Mass eccentricity [microns]
                U = Unbalance [gr•mm]
                M = Rotor mass [kg]

According to ISO 1940 standards, all rotors are classified (grouped), depending on their balancing requirements (look at following table). Balancing quality G is a number which defines the balancing accuracy required; for instance G = 2,5 means that a fine balancing is required, G = 6,3 means that a normal balancing is accepted.

Please note that the measuring unit for  G is mm/s, because this value represents the vibration speed assumed by the body rotating freely in the space at the real service speed.

The same value of vibration speed ( G=mm/s) is achieved by the rotor, when it rotates mounted on a soft bearing machine at service speed.

Balancing can be a very complex subject and I would be delighted to hear from anyone experiencing unbalance who is in need of technical advice.

Cemb Hofmann is the UK's leading specialist in balancing - from balancing machine sales to a world-class sub contract dynamic balancing service.

Visit www.cembhofmann.co.uk today for more information or call us on 0161 872 3123 to speak to a member of our team today.