Safety tests of grinding wheels

Grinding wheels for use on hand-held machines: Verification of side load capacity

Safe abrasive products must be able to withstand the forces and loads that are to be expected both under normal conditions of use and in the event of misuse. The resistance to deformation and bursting depends on various factors such as material, geometry, type and duration of stress and environmental conditions.

In hand-held applications, in addition to the tangential forces acting on the periphery of the abrasive product, there are always also lateral (axial) stresses acting on the grinding wheel. With products approved for side grinding, these stresses are completely normal. However, a slight tilt or a curved cut is enough to cause enormous lateral loads even on cut-off wheels that are only approved for straight cuts.

The EN safety standard for bonded abrasive products (EN 12413) therefore requires that certain abrasive products have sufficient side load capacity in addition to a safety factor against bursting. The side load capacity is determined by the single-point side load and the impact test (working capacity). The requirements generally apply to resin bonded grinding wheels for use on hand-held machines. EN 12413 specifies required values for the single-point side load and the working capacity depending on the type of grinding wheel. The side load capacity is verified by the single-point side load test and the impact test.

The single-point side load test is carried out on both cutting-off and grinding wheels. The abrasive product rotates at maximum operating speed. Then a pressure roll is fed at a constant feed rate vertically to the axis of rotation against the grinding wheel and loads it until rupture or up to the required value. During the test, the force acting on the pressure roll in the feed direction is measured as the single-point side load.

If the grinding wheels reach the required value without breaking, the verification of the single-point side load is considered as provided. The grinding wheels used for the test must then generally be destroyed.

In order to avoid misinterpretations regarding the side load capacity, the tests should always be carried out with pressure rolls in mint condition. The choice of material for the pressure roll is also important. Here, free-cutting steels or also hardenable aluminium alloys are used preferably. The pressure roll must be mounted in such a way that it can rotate freely without jamming on contact with the grinding wheel. The test chamber must be sufficiently large. In particular, the grinding wheel must not rest against the rear wall of the test chamber as a result of the side load. Rather, it must be able to deform freely.

In the impact test, the resistance to a shock-like, lateral impact load is determined. The grinding wheel to be tested is clamped onto a rotating spindle by means of flanges and then accelerated to its maximum operating speed. The rotating grinding wheel is then loaded laterally with one or more successive impacts supplied by means of a pendulum testing machine until rupture. The impact energy can be adjusted by changing the drop height of the striker as well as by attaching additional masses. Details of the test procedure can be found in EN 12413.

Verification of working capacity is considered as provided if no damage is visible to the naked eye at the required value. The following illustrations show typical damage patterns. The grinding wheels used for the test must generally be destroyed afterwards.

Types of damage in impact tests

Type c)   Breakage at bore area; the example pictures were obtained at the IFA and show different types of damage

Type d)    Breakage into several pieces; the example picture was obtained at the IFA

In addition to regular performance checks, it is important to calibrate the test equipment at certain intervals. It is recommended to have the pendulum striker calibrated by an external authority at least once a year.