Shure 61B, 62B User Guide

DATA SHEET No. 163A

DATE: June 1942

S U B J E C T : Models 61B and 62B

Vibration Pickups

MODELS 61B AND 62B VIBRATION PICKUPS

GENERAL:	Models 61B and	62B Vibration	Pickups
	are piezoelectric instruments for
	the measurement	and analysis	of vi-

brations in industrial fields. The piezoelectric element is a grafoil bimorph Rochelle salt crystal unit with special-process moisture-proofing. The crystal element is inertia actuated, which principle eliminates the need of a stationary reference body in vibration measurements. The isolated mounting used in this type of actuation gives a maximum protection against breakage of the crystal. The assembly is enclosed in a cast aluminum case.

The output voltage Is proportional to the vibration due to acceleration throughout the linear range of the instruments, giving a relative measure of the stresses set up by the vibratory motion. Model 61B has a linear voltage-acceleration characteristic up to approximately 1000 cycles per second, a frequency range which is suitable for general vibration studies. Model 62B has a voltage output about four times that of the 61B and a linear characteristic extending to 500 cycles per second. The latter model is recommended for applications such as the direct energizing of headphones or oscilloscopes, which require a relatively high output voltage. Both instruments are responsive beyond the linear range, to frequencies up to approximately 3000 cycles per second. Modification of the frequency characteristics is possible through use of simple circuits, resulting In a greatly increased flexibility of the devices.

Each instrument is equipped with a complete set of adapters Including an extension rod, ball-tip, point-tip, and mounting bracket.

APPLICATIONS : Models 61B and 62B Vibration Pickups have innumerable applications in scientific research, industrial de-

sign, inspection, and In many other fields. Some of these applications are suggested below:

Industrial Research and Design: Determination of optimum shapes and material distribution in high-speed rotating members, motor windings, and hydraulic equipment. Research in vibration isolation materials, construction of vibration-proof rooms, vault protective systems, etc. Noise reduction of automobile and airplane structures, gearing, bearings, etc.

Manufacturing and Inspection: Balancing of rotors, adjustment of gears and impact devices, uniformity of air-gap pull, smoothness of surfaces; vibration in machinery, building structures, transmission lines, antenna towers, etc.

Servicing and Surveying: Location and elimination of sources of vibration, loose pistons and bearings; servicing refrigerators and air-conditioning systems; isolation of machinery, tracing leaks in water-pipes, tanks, and hydraulic structures, geodetic surveying for oil and mineral strata, etc.

INSTALLATION: Each pickup is equipped with a stationary mounting bracket and a screw for the 1/4" - 28 thread in the

pickup shank. Wherever it can be used conveniently, the screw itself will give sufficient support. Other accessories included am a ball-tip, point-tip, and 8"

Fig. 1. Illustration of Models 61B and 62B Vibration Pickups and accessories.

(Approximately 1/3 actual size)

extension rod. Special. setups for production testing may be easily constructed, but care should be taken to transmit vibrations approximately along the axis of the pickup shank.

Each instrument is provided with a 7' single conductor rubber-jacketed shielded cable. If necessary, the length of cable can be increased considerably without excessive output loss. Low-capacity high in- sulation-resistance shielded cable should be used. Shielded crystal microphone cable having a capacity of 25 to 50 micromicrofarads per foot, and a leakage resistance of 200 megohm-feet of more, is recommended. All joints should be well-shielded to avoid hum pickup.

The crystal enclosure is entirely water-tight, but the cable connector box, while reasonably moistureproof, will not withstand immersion in water. (For underwater work specially constructed Instruments are available). The pickup should not be subjected to extremely violent shocks or vibrations, nor allowed to attain temperatures in excess of 125° F (51.7° C). Extension rods of sufficient length will usually permit low operating temperatures near hot machinery.

CORNECTIONS:	The	pickup should be connected to
	the	grid circuit of a vacuum tube
	across a load resistance of ½ megohm

or more. The green-coded conductor should be connected to the "high" or grid side of the amplifier input, and the shield (black conductor) should be connected to the ground or chassis of the amplifier: (See Fig. 2).

The full output of the pickup may reach peaks as high as 20 volts or more when measuring vibrations of large amplitude. Under such circumstances, the device should not be connected directly to the grid of the tube without the use of a volume control or some other type of voltage divider. A convenient way to attenuate the output voltage and avoid tube overloading is to shunt the pickup terminals with a condenser as indicated by the dotted lines in Fig. 2. A 0.05 microfarad shunt condenser will usually reduce the peak

No. 163

DATA SHEET

Fig. 2. Recommended amplifier connection for	Fig. 3. Suggested indicating arrangements
crystal vibration pickups.	for vibration pickups.

voltages to a safe value without affecting the frequency characteristic. In measuring vibrations of moderate amplitudes, this precaution may not be necessary.

For most applications, a low or medium gain amplifier will be satisfactory. The pickup may be very conveniently connected directly to conventional type cathode-ray oscilloscopes. In cases where the output Is sufficiently high, the vibration pickup may be used directly with high-impedance headphones, the crystal type being especially suitable.

The use of transformers is not generally advisable in conjunction with crystal devices.

OPERATION:	In	addition	to permanent mounting
	arrangements, the pickup may be held
	in	the hand.	The point-tip may be

used for point-to-point exploration, while the roundtip is useful in checking surfaces for smoothness. Remote parts of machinery may be reached through use of the extension rod.

The maximum output of the pickup is obtained when the direction of the displacement coincides with the axis of the pickup shank. When the extension rod is used, the readings obtained will represent the component of vibration along the line of action of the rod.

In hand exploration and test set-ups, only enough pressure should be applied to the pickup to keep it

Fig. 4. Typical frequency-response characteristics of Model 61B Vibration Pickup.

(Applies to Fig. 2 without Condenser)

firmly In place. Excessive pressure may alter the amplitude of vibration observed and give erroneous readings. The instrument should never be subjected to exceptionally violent vibrations.

The block schematic diagrams of Fig. 3 give some suggested operating arrangements of the pickup with head-phones, oscilloscope, wave analyzer, or amplifier and meter. Since the forces developed in the inertiaactuated crystal are in close relationship to the stresses set up in vibrating bodies, the amplifier meter readings will be roughly indicative of the average vibrational stress. A calibrated cathode-ray oscilloscope In conjunction with the pickup will serve as a convenient means by which the values of vibrational components may be estimated from visual observations.

The instruments should not be allowed to attain temperatures exceeding 125° F. An extension rod of sufficient length may be used to keep the pickup at a safe distance from hot devices.

FREQUENCY Frequency response curves for conCHARACTERISTICS: stant amplitude vibration are shown in Figs. 4 and 5. The voltage output of the 61B Pickup is very nearly proportional to

vibration acceleration up to approximately 1000 cycles per second, and has a value of about 5 millivolts per one-millionth inch total displacement at 250 cycles per second. The voltage output of the 62B is approximately 20 millivolts under the same conditions, and is essentially proportional to acceleration up to about 500 cycles per second. The total frequency range of both pickups extends to about 3000 cycles per second.

The internal impedance of the instrument is approximately equivalent to that of a .005 microfarad condenser and hence the low frequency characteristic is dependent upon the terminal resistance employed, as Indicated in Figs. 4 and 5. The reduction in lowfrequency response obtained by connecting the pickup to a low terminal resistance is a decided advantage in some cases. For general applications, a terminal resistance of 1-megohm is suitable. For studying very low frequency vibrations, a 3 to 5 megohm termination should be used.

When the pickup is used with the extension rod, the actual motion transmitted to the pickup may be altered, depending upon the particular conditions. As a general rule, frequencies above 1000 cycles per second will be attenuated. Since frequecies below 1000 cycles per second predominate in industrial problems, the effect or the rod is usually negligible.

Frequency characteristics of the pickup may be modified, if desired, by means of simple electrocal networks described in the Appendix, page 3. In all cases, the complete circuit, Including the network Itself, should be fully shielded to prevent hum pickup.

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