SHAFT HORSE POWER METER (TORSION METER)

 

The Shaft Horse Power Meter measures shaft torque and thrust using strain gauge technology. The instrument consists of an aluminium ring clamped onto the shaft; a stationary unit located next to the shaft and a junction box for the signal and power connections.

 

The shaft ring contains electronic circuitry for signal processing and transmission, and also serves as protection for the strain gauge(s) which are glued onto the shaft surface.

Values for torque and thrust are transferred as frequency modulated signals to the stationary unit through contact free transmission.

 

Shaft rotation is measured by the sensing of magnets or steel studs on the shaft ring. Shaft power and total energy are calculated by signal processor in the stationary unit. The following propulsion data are recorded by the Shaft Horse Power Sensor and presented on the Display Unit:

Rpm, Power, Torque, Thrust, Total Energy, Total revolutions

 

The Shaft power (Sp) is calculated directly from the shaft speed and measured torque. The general formula is:

 

Shaft Horsepower (Sp) Formula 

Sp = 2.Pi.N.T/60kW
T- torque which is measured by torsion meter (kNm)
N-revolution per minute of the shaft speed (RPM)

Principle of Measurement

1.1   Torsion Sensor

The sensor used for the “Shaft Horse Power Meter” detects microscopic torsion of the shaft through the change in vibration frequency of amorphous alloy sensor strip.

The basic principle of torsion sensor is that the change in vibration frequency of a stretched elastic strip depends on tension in the strip. In practice, an amorphous alloy strip is stretched between 2 supporting blocks and two sets of electromagnets are located closely to the strip. One of these electromagnets is an exciter and another is a pick-up. When the sensor strip is caused to oscillate by electromagnet, it vibrates at its natural frequency, which is dependent on tension in the strips as the formula mentioned below.

If a displacement is induced in the torsion detector rings on which the sensor is mounted, the distance between two supporting blocks will be changed. Accordingly, tension in the strip and its vibration frequency also will be changed. Thus, the change in vibration frequency of the sensor is a measure of the strain or stress applied to the sensor. Actually, the vibration frequency (Hz) is proportional to square root of tension “T”, the square of vibration frequency (Hz^2) is proportional to the displacement.

 

The calibration constant, the relationship displacement and the change in square of vibration frequency of every sensor is individually determined in unit of cm/ Hz^2 before delivery. For instance, calibration constant of a sensor is given as 0.587 × 10^-7cm/Hz^2

 

1.2 Torsion Detector Ring

The torsion detector ring comprises of two sets of split rings and data transmission system. The split rings are clamped on the shaft independently at a distance of 70mm and two torsion sensors are mounted across the fitting blocks of both rings.

When a torsional moment is applied to a shaft, the positional change between two rings causes to change the tension of sensor strip. Hence the amount of torque applied is obtainable through the change in sensor frequency and some constant values such as calibration constant of sensor, diameter and modulus of elasticity of the shaft and so forth.

Two sensors are used to exclude the influence of temperature change and the vibration frequency by the centrifugal force.

When added the torque, two sensors are installed in the direction where the frequency rises and the decreasing direction. Therefore, the mean value of the frequency is counterbalanced by requesting two sensors because frequency increases in the other sensor, and decreases on the other hand.

The sensor signals are modulated by transmitter units and transferred to receiving antenna without contact by transmitter units. For the power supply to transmitter units, a contactless power supply unit is provided facing to the torsion detector ring. The electromotive power generated between this primary coil unit and secondary coil wound inside torsion detector ring is boosted and rectified to D.C. 8 volt.

The shaft speed is calculated by counting the pulse signals, which is generated by proximity switch located on power supply unit. And one detecting plate mounted on the ring.

1.3 Digital Computing Unit

The computation is performed by an 8-bit microprocessor unit. The squares of two sensor signals (Hz^2), shaft speed signal, K value and zero point are substituted to the formula mentioned below to calculate shaft horsepower, shaft speed and torque. K value is a constant to be set in advance using keyboard and zero point is determined by auto zero-point scanning function.

Since sensor signals and shaft speed signal are integrated for 5 seconds, then applied to calculation formula, measured values are updated every 5 seconds. For indication, a 4-digit dual L.E.D. display unit with dimmer and selector switch is provided. Upper display unit indicates shaft horsepower constantly and the lower is for selective indications of shaft speed, torque and average shaft horsepower.

 

Shaft Horsepower Averaging Function

This function stores shaft horsepower data obtained every 5 seconds for a specified

time and calculates running average. The time for averaging is programmable between

1 and 240 minutes at the increment of 1 minute to a preset switch.

 

Auto Zero Point Scanning Function

This function automatically determines actual zero point (neutral point of torque) by mean sensor data (Hz^2) while slow turning the shaft ahead and astern each one rotation in order to cancel static torque caused by friction at stern tube seal and intermediate bearings.

 

2. Formula for Shaft Horse Power

 

               = {(CH1 – CH2) + (Z2 – Z1)} x K/2 x RPM x 0.7355

              = {(CH1 – CH2) + ZERO} x K/2 x RPM x 0.7355

 

Where: CH1 = Data of sensor 1 under load (Hz^2)

Z1 = Data of sensor 1 under no load (Hz^2)

CH2 = Data of sensor 2 under load (Hz^2)

Z2 = Data of sensor 2 under no load (Hz^2)

Zero= Zero point = (Z2 – Z1)

K = Calculation constant

 

Where: Ip = Polar moment of inertia of the shaft, π D^4/32

G = Shear modulus of elasticity of shaft

R = Distance between sensor strip and shaft centre

S = Measuring span of detector ring

Cm = Mean calibration constant of torsion sensors

71620 = Torque → horsepower conversion constant