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.

The Common & Frequent Words of Balancing

Balancing is a complex and tricky artform - producing a technical language that is riddled with certain phrases and words that would confuse many customers in need of balancing advice.

Below is an explanation of two of the most common used terms.

Static balancing :  Unbalance measuring and correction is carried out in one plane only.

Dynamic balancing :  Unbalance measuring and correction is carried out in two different planes.

Correction planes :  Is the section (plane) where unbalance correction is performed by adding or removing mass.

At Cemb Hofmann we pride ourselves in the delivery of first class balancing services - from high quality machine sales to a world-class sub contract balancing service.

To find out more visit www.cembhofmann.co.uk or call our team today on 0161 872 3123

Balancing Speed: A Comprehensive Explanation

The unbalance of a rotor is caused by the radial distribution of its masses along its axis of rotation.

The consequence is that if the rotor is rigid, and this means that the values and relative positions of its masses do not change, the unbalance does not change with the speed. In a rigid rotor the operating speed does not modify mass distribution and consequently has no influence on the unbalance.

By adding a 20 gr mass at a defined radial position on a perfectly balanced disc an unbalance is generated. This unbalance does not change with the speed, it is just necessary to remove the added 20 gr mass independently of rotor speed.

For rigid rotors the balancing speed does not need to be specified because it is related only to machine sensitivity and not to the rotor unbalance which is under measurement.

Modern hard bearing balancing machines have the capability to measure the dynamic unbalance starting from 70 RPM.

The unbalance effect (centrifugal force) increases with speed, the electric signal increases at the same time and so machine sensitivity tends to increase because of a better ratio signal to noise.
Depending on the model and manufacturer optimum sensitivity values are obtainable starting from 400 to 600 RPM.

Note: don’t get confused between the cause (unbalance) with its effect (centrifugal force or vibration).

The effect increases with the speed while the cause (unbalance) in a rigid body does not change.

For further information about any aspect of balancing please visit www.cembhofmann.co.uk or call our expert team today on 0161872 3123.

Dynamic Balancing

 

Dynamic balancing a rotor means to reduce its dynamic unbalance to zero or to an acceptable level.

This process will eliminate costly and sometimes irrepairable damage to the unit and is absolutely essential to the maintenance and quality of rotating parts.

 

The dynamic unbalance is by definition:

 

 

So it is necessary to operate on two different planes.

 

Since the dynamic unbalance equivalent ot the total unbalance Ut can be calculated with reference to two arbitrary planes, the consequence is that thte two balancing planes (where material can be added or removed) can be chosen.

 

The above is valid for rigid rotors where mass distribution (local unbalances) does not vary with the speed.
 

 

In order to balance with the minimum effort two rules are valid:

 

 

1)  Choose balancing planes as far apart as possible

 

2)  Choose a balancing radius as large as possible

 

 

Note: by the dynamic balancing on two different planes the total unbalance (set of local unbalances) is not reduced to zero; only the dynamic unbalance (on two planes ) equivalent to the total unbalance Ut is reduced to zero.

 

 

 Discover more about dynamic balancing by visiting the Cemb Hofmann resource centre or call our expert balancing team on 0161 872 3123.

 

Couple Unbalance

A Definition of Couple Unbalance

The total unbalance is called couple unbalance if the equivalent unbalance is made by two vectors placed on two different planes having equal values and opposite directions.

 (The axis of inertia cuts the axis of rotation passing through the centre of mass)

 

 

Of course values Us and Ud (unbalance value in the two sections) are equal.

 

For example: if the declared couple unbalance value is 6000 gr.cm.cm and the distance between the two balancing planes is  15 cm, then the unbalance per plane is  6000/15 =400gr.cm (4000 g.mm ). If the balancing radius on each plane is 20 cm, then the unbalance per plane is 400/20=20grams (the two unbalances on each plane are equal in value but opposite in the angle position)

 

For more information on balancing or sub contract dynamic balancing services, please call our expert today on 0161 872 3123 or visit the Cemb Hofmann UK site.