Cable Tensiometers are hand-held instruments used to measure the tension of a cable. They are often used in the aviation industry during cable rigging in aircraft by aircraft manufacturers and for periodic cable maintenance by aircraft maintenance technicians.
Cables are used in an aircraft to connect the flight controls in the cockpit with the movable parts on the aircraft’s wings and stabilizers, such as the ailerons, elevators and rudder. Cable rigging involves the setting of the travel limits and cable tensions of the cables in the aircraft control system.
Cable Tensiometers are typically used on aircraft cables that have 7 x 7 or 7 x 19 stranding in the cables and in diameter sizes of between 1/16 through 3/8 of an inch.
Cable Tensiometer Types
Cable Tensiometers come in two basic types, direct-reading and indirect-reading. Both types are designed to measure various types of cable, cable sizes and cable tensions.
Direct reading tensiometers will have a dial or digital readout located on the instrument to give the actual tension indication. The dial will have specific graduations that indicate the cable tensions for each of the various cable sizes the instrument can measure.
Indirect reading tensiometers contain a larger dial with a scale that reads between 0 and 100. A calibration card or graph is supplied with these units that shows a conversion from the dial reading to the pounds of tension for the different cable sizes to be measured.
Use of Tensiometer Risers
The indirect reading tensiometers require the use of several different removable ‘risers’ that are used to measure different cable sizes. Each riser is of a different height which makes it possible to accurately measure tensions of different cable sizes. Each riser has a specific identifying number and is easily inserted onto a pin on the plunger of the tensiometer.
The specific riser needed for the appropriate cable size is listed on the calibration card for each tensiometer. The correct riser must be used for the cable size being measured or incorrect readings will occur and damage to the tensiometer may result.
How Tensiometers Measure Cable Tension
Cable Tensiometers work by measuring the amount of force required to make the cable move between two hardened steel anvils set a specific distance apart. A jaw with the two anvils is located at the top of the instrument and a plunger with the riser is located between the two anvils.
To measure the tension on a cable, the cable to be tested is placed between the two anvils and the tensiometer handle or trigger is closed. The plunger will move up and the riser will contact the cable at a right angle to the clamping points on the anvils. The force that the riser contacts the cable is indicated by the pointer on the dial.
In some cases when measuring aircraft cables, the position of the tensiometer may make it difficult to view the tensiometer dial. A pointer lock is built into the tensiometer to view the cable tension measurement when the dial is facing away from the operator.
Cable Tensiometer Calibration Process
Calibration of Cable Tensiometers is usually performed by hanging accurate dead weights from various cables. In many cases, hydraulic rams or pull test machines can be used to apply the varying forces for the calibration, and load cells can be used to accurately measure the force on the cable. The position of the tensiometer does not affect the readings so it does not matter whether the calibration cables are placed in a horizontal or vertical position.
The tensiometer must be calibrated on the same type and size of cables on which the instrument is designed to be used. Typically, at least five readings are taken on each different cable to cover the full measurement range of the cable. The individual readings are recorded on the calibration card.
The calibration card is created for each specific tensiometer. The serial number and calibration date are noted on the calibration card for each unit. In many cases, a graphical chart will be generated from the calibration data. The chart makes it easier for the maintenance technicians to determine the correct reading between the calibration points.
Wear to the anvils and risers can change the calibration readings so tensiometers need to be calibrated regularly to maintain their accuracy. Tensiometers should be calibrated at a minimum of one year or sooner depending on how often the tensiometer is used.
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