IEC60068-2-75 Calibration Device for Spring Operated Impact Hammer
Conforming to the standard requirements of IEC60068-2-75 attachment B, this test device is designed and manufactured for the calibration of the spring hammer.
Standard: IEC 60068-2-63 Annex A, IEC 60068-2-75 Annex B
Application: This device is intended to verify the accuracy of the spring hammer calibration device.
Test sample: Calibration Device for Spring operated impact hammers
Feature: Comply with the requirement of standard IEC60068-2-63 and IEC60068-2-75
|Impact element for calibration
|1J impact energy achieved with a height of fall of 408±1mm
Procedure for the calibration of spring hammers
B.1 Principle of calibration
The principle of this calibration procedure is to compare the energy provided by a spring hammer, which is difficult to measure directly, to the energy of a pendulum, calculated from its mass and height of fall.
B.2 Construction of the calibration device
The assembled calibration device is shown in figure B.1. Apart from the frame, the main parts are a bearing “a”, a drag pointer “b ”, a pendulum “c”, a release base “d” and a release device “e”.
The main part of the calibration device is the pendulum “c” shown in figure B.2. To the lower end of this pendulum is fixed a steel spring with the details shown in figure B.3. The spring is of spring steel, requiring no special treatment, and is rigidly fixed to the pendulum “c”.
Figure B.4 shows some parts on a large scale
It should be noted that this spring is designed for calibrating spring hammers having characteristics as defined in table 1 for energy values equal to or less than 1 J. For calibrating spring hammers having characteristics as defined for 2 J, the spring of the pendulum of the calibrating device would need to be of a different design.
In order to obtain suitable friction characteristics of the pointer, a piece of thick woven cloth is placed between the metal surfaces of the bearing, the piano wires being bent in such a way that a small force is exerted against the cloth.
Because the release device is removed during the calibration of the calibration device, the release device is fixed to the release base by means of screws.
B.3 Method of calibration of the calibration device
The calibration of the calibration device is effected by using a calibration striking element “g” taken from a spring-hammer, as shown in figure B.5. Before calibration, the release device is removed from the calibration device.
The calibration striking element is suspended by four linen threads “h” from suspension points situated in a horizontal plane, 2000 mm above the point of contact between the pendulum and the calibration striking element when the latter is in its rest position. The calibration striking element is allowed to swing against the pendulum and the point of contact under dynamic conditions, point “k”, shall be not more than 1 mm below the point of contact in the rest position. The suspension points are then raised over a distance equal to the difference between both contact points.
When the suspension system is adjusted, the axis of the calibration striking element “g” shall be at right angles to the impact surface of the pendulum “c” and the calibration striking element shall be horizontal at the moment of impact.
When the calibration striking element is in its rest position, the calibration device is placed so that point “k” is positioned exactly at the head of the calibration striking element.
To obtain reliable results, the calibration device is rigidly fixed to a massive support, for example to a structural part of a building.
The height of fall is measured at the centre of gravity of the calibration striking element and the measurement can be facilitated by using a liquid level device consisting of two glass tubes “j” which are interconnected by means of a flexible hose. One of the glass tubes is fixed and provided with a scale “I”.
The calibration striking element may be held in its upper position by means of a thin thread “m” which, when ruptured, causes the release of the calibration striking element.
For scaling the calibration device, a circle is drawn on the scale plate, the centre of this circle coinciding with the bearing of the pendulum and its radius being such that the circle extends to the drag pointer. On this circle, the zero point 0 J shown in figure B.6 is marked at the point indicated by the drag pointer when the latter is brought into contact with the pendulum in the rest position.
The calibration is made with an impact energy of 1 J, which is achieved with a height of fall of 408 mm ± 1 mm, with a calibration striking element of 250 g.
The point on the scale plate corresponding to 1 J is obtained by allowing the suspended calibration striking element to swing against the point “k” on the spring of the pendulum. After hitting the pendulum, the calibration striking element shall not move. The operation is repeated at least 10 times and the 1 J point is the average of the indications of the drag pointer.
The other pints of the scale are then determined as follows:
a) A straight line is drawn through the centre of the circle and the 0 J point;
b) The orthogonal projection of the 1 J point on this line is indicated by P;
c) The distance between the points 0 J and P is divided into 10 equal parts;
d) Through each dividing point, a line is drawn perpendicular to the line 0 J-P;
e) The intersections between these lines and the circle correspond to values of impact energy equal to 0.1 J; 0.2 J; up to 0.9 J.
The same principle can be used for extending the scale beyond the 1 J point. The division of the scale plate “f” is shown in figure B.6.
B.4 Use of the calibration device
The spring hammer to be calibrated is put in the release base and is then operated three times by means of the release device; it shall not be released manually.
For each operation, the striking element of the spring hammer to be calibrated is turned in a different position. The average value of the three readings on the calibration device is taken to be the actual value of the impact energy of the specimen.