The Load Guard is intended for use as an overload guard in cranes and lifts, for the automatic tensioning of belt conveyors and for other automatic power-sensing/load control.

The movement of the pull rod operates the microswitches through spring-loaded contact points. The power-absorbing element consists of specially made Belleville washers dimensioned to resist fatigue. The spring washers cannot be overloaded.

The load guard is fully pressure tight. Each instrument is tested under pressure. Externally it is zinc coated and white chromated and the terminal box is polyester-lacquered. If the load guard is to be used in a very corrosive environment, the complete instrument can be polyester-lacquered.

Safety factor 5:1 (guaranteed strength against breakage at least 5 times the max. capacity).

The load guard can be overloaded by 100% without interfering with the switch setting. As the transmission parts between the pull rod and the microswitch are spring-loaded, the load guard can withstand severe shock loads and sudden unloading.

The pull rod and upper bracket of the load guards types IMB-IG are made of drop forged steel SIS 2174 (St 52-3N acc. to DIN 17100, 50 D acc. to BS 4360) which has guaranteed impact strength down to -20°C.

The setting of the microswitch is sealed by an aluminium plate over the switch, on which the set value is engraved. The plate is screwed and glued to the housing with a 2-component adhesive. This plate cannot be removed without torch heating to about +80°C.

The load guard with one or two microswitches is supplied with alternative contact connections for each microswitch. Load guards with three, four or five microswitches have either one breaking or closing function on each switch. These are normally set for the breaking function (closed contacts at rest) and for rising loads, unless otherwise ordered. When setting for falling load, e.g. control on the slack line, the microswitch is connected for the closing function (open contact at rest).

If the load guard is to be used for safety application, the breaking function should always be used.

The microswitches have gold-plated contacts, which make them suitable for operation on low current/voltage (under 10 V and/or 60 mA).

The difference between the switching value on rising and falling loads is approximately 3% of the max. capacity of the instrument. This hysteresis is somewhat reduced at lower loads and increases slightly at higher load values. To avoid "chattering" of the contacts and contactors if the load should start to sway, a time relay to delay the reclosing of the hoisting circuit may be installed. When setting for falling loads, special microswitches with a reduced hysteresis are used.

The switch values are set by the factory to the required level.

The Piab Dynamometer can be equipped with a built-on adjustable contact head. Using a microswitch, which is directly influenced by the pull rod, the dynamometer gives an electrical contact function at a previously set value. For the types MA-O the extension to the body length is about 3.15/16" (100 mm), for Q about 4.1/3" (110 mm), for S about 3.5/64" (78 mm), for U and G about 3.5/8" (95 mm) and for the type E about 4.1/64" (102 mm).

For a dynamometer with 1100 lb. or 2200 lb. capacity, the types MA and MB are used. These have spring assemblies for 1100 lb. or 2200 lb., arranged in type M housings.

The dynamometers MA-O can be equipped with 1-3 and the types Q-E with 1-5 microswitches. The electrical connections are made in the terminal box to a numbered connection block.

Repeatability: ± 0.6% of the max. capacity.

Max. Contact Load: 250 VAC, 500 VA, 3A.

Max. Cable Area: 4 mm²

The Mechanical Life of the Microswitch: 20 mill. cycles.