Auto-Balancing cuts vibration by half
Using an old and well-know principle, SKF has developed a compact Auto-Balancing Unit for the new Atlas Copco Turbo Grinder. The unit continuously cuts the imbalance of the grinding discs to a minimum during operation, reducing vibrations and risk of vibration injuries.That operators of hand-held grinders are exposed to high vibration levels is a problem gaining increasing attention. Within the Common Market, new CE regulations regarding a vibration declaration for hand-held tools were introduced from January 1, 1995. Grinders now have to be marked according to vibration class. The time an operator can use a machine without increased health risk is limited according to these classes (see figure 1). The vibration level is measured as a frequency weighted average acceleration, using a standard aluminium disc with holes to simulate true grinding disc imbalance. If this level is less than 2.5 m/s2 the tool can be used a whole working day without restriction. Above this, a time limit is recommended.
Using an old and well-know principle, SKF has developed a compact Auto-Balancing Unit for the new Atlas Copco Turbo Grinder. The unit continuously cuts the imbalance of the grinding discs to a minimum during operation, reducing vibrations and risk of vibration injuries.That operators of hand-held grinders are exposed to high vibration levels is a problem gaining increasing attention. Within the Common Market, new CE regulations regarding a vibration declaration for hand-held tools were introduced from January 1, 1995. Grinders now have to be marked according to vibration class. The time an operator can use a machine without increased health risk is limited according to these classes (see figure 1). The vibration level is measured as a frequency weighted average acceleration, using a standard aluminium disc with holes to simulate true grinding disc imbalance. If this level is less than 2.5 m/s2 the tool can be used a whole working day without restriction. Above this, a time limit is recommended.
For each class, usage time has been halved.
Angle grinders can normally be found in vibration classes 4 or 5. An Auto-Balancing Unit typically reduces vibrations by three classes, and further if additional design changes are incorporated.
When Atlas Copco started to develop the new pneumatic Turbo Grinder GTG40 back in 1989, low vibration level was one of the most important requirements. This demand, however, contradicted the requirement of minimum machine weight. Low weight gives low inertia which makes the system sensitive to imbalance forces. Various solutions including heavy handles or housings were tested and rejected. Efforts to improve disc balancing quality also proved fruitless. Not only does the disc balance vary between different discs, it also varies for each disc during use, due to wear.
By incorporating the new Auto-Balancing Unit from SKF, both demands could be met; low vibration levels and low weight were achieved.
The unit is based on an old (the oldest patent is from the 20s) and well-known principle of freely movable balls that in an over-critical phase counteract a rotating system’s imbalance (see figure 2 and 3).
In the early 90s, additional pioneering work on balancing of hand-held grinders was done by Hans Lindell at the Swedish Institute of Production Engineering Research (IVF) on behalf of The Swedish Work Environment Fund. The friction between the balls and raceway and other elementary parameters was subject to research at SKF Nova AB in Gothenburg during 1992. It was found that ball bearing technology in terms of precision, surfaces, hardness and lubrication was crucial for high-performance balancing. This means that SKF has a unique ability to manufacture Auto-Balancing Units with just the right properties.
Contacts were established with Atlas Copco regarding their new grinder and a joint project including SKF Spherical Roller Bearing Division in Gothenburg, SKF Nova AB and Atlas Copco was started in late 1992. Tests of SKF-made prototypes showed positive results from the beginning. The work resulted in a first series delivery of Auto-Balancing Units from the SKF factory in November 1993.
More than a year after its introduction, several advantages of the Auto-Balancing Unit are evident:
- Substantially reduced vibration levels with maintained low weight.
According to the vibration test ISO 8662-4, the Turbo Grinder equipped with Auto-Balancing cuts vibration by more than half, increasing eight-fold the time of operation. The Atlas Copco Turbo Grinder operates in the zone of unrestricted use below 2.5 m/s2.
- Balancing continuous during operation.
Although imbalance in a grinding disc varies during operation due to wear, good balancing results are always obtained, since the balls instantly and continuously compensate for the varying imbalance.
- As operator comfort is improved there’s better grinding quality and higher efficiency.
- Longer life of the machine as an effect of reduced loads.
The market reception, according to Atlas Copco, has been overwhelming. The combination of very high grinder effect, low weight and low vibration level fulfils the operator requirements perfectly. This one-of-a-kind machine has substantially increased sales.
For SKF it has provided the opportunity to offer a new type of product based on our core technology; rolling bearings. As a part of our strategic partnership concept, SKF has been involved in the Atlas Copco project from an early stage. This has enabled SKF to solve a critical problem of a hand-held grinder.
The auto-balancing principle
In a grinding disc with imbalance, the mass centre (MC) and the geometrical centre (GC) do not coincide. Operating at over-critical speed as in all hand-held grinders, the disc rotates around its mass centre. If an Auto-Balancing Unit is fitted on the drive spindle next to the grinding disc, the balls inside the unit are affected by the following forces:
FC= Centrifugal force from the mass centre
FN= Normal force perpendicular to the raceway contact
Since the forces do not counteract exactly, a resulting force, FR, is developed, making the balls move along the raceway. The balls will stop when a state of equilibrium is reached. By utilising the balls’ mass, the system is now balanced. The MC and the GC are the same point. FN and FC counteract exactly. The balancing has taken less than 1/10 second.
The whole angle grinder vibration spectrum is dominated by the disc imbalance, showing up as a peak at operating speed. By cutting the imbalance peak by means of an Auto-Balancing Unit, the total vibration level is substantially reduced.
In the case of big disc imbalance, the balls will all be positioned together on one side of the unit. Small imbalance means the balls are spread out but biased towards one side. The total balancing capacity of an Auto-Balancing Unit depends on the number and size of balls and the diameter of the raceway.
If imbalance changes during operation, as is the case for grinding discs, the balls immediately and continuously correct their positions.
The resulting force FR that moves the balls is very small. Low friction between the balls and raceways is therefore essential for good balancing. Bearing manufacturing technology in terms of surfaces, hardness, precision and lubrication is necessary to achieve positive results.
Jonas Nilsagård
and Helene Richmond,
SKF Spherical Roller Bearing Division,
Gothenburg, Sweden