Glossary

Acceleration – Rate of change of velocity with time. Usually along a specified axis, usually expressed in “g” or gravitational units. It may refer to angular motion.

Accelerometer – The part of the Balancing System that provides a vibration signal to the Controller and allows for processing of the level of unbalance in the rotating equipment.

Adhesive – Substance for joining or bonding components together.

Adhesion – The state of two components being joined or bonded together.

Amplifier – The part of the Balancing System that provides modulated power to the Coil Assembly at the command of the Controller. Depending on the system configuration the Amplifier may or may not be an integral part of the Controller.

Amplitude – The maximum displacement from its zero value position.

Balance Plane – A plane perpendicular to the fan shaft defined by the location of the Accelerometer.

Balancer Assembly – The rotating portion of the Balancing System that houses the counterweight rotors used to compensate for the unbalance in the rotating equipment.

Balancer/Coil Assembly – Consists of the Balancer Assembly and Coil Assembly.

Balancing System – A system that automatically compensates for rotating unbalance while the unit is running. This system includes the Controller, Amplifier, Human Machine Interface (HMI), Balancer Assembly(ies), Coil Assembly(ies), Cables, Accelerometer(s) and appropriate software.

Bracket Assembly – The mechanism used to mount the Coil Assembly to the fan’s bearing pedestal. This assembly must be rigid enough to prevent movement of the Coil Assembly with the Balancing System is in operation. The Bracket Assembly is typically comprised of an aluminum plate, which the Coil Assembly mounts to as well as a series of steel angle pieces that are used to attach the aluminum plate to the bearing pedestal and provide structural support.

Carbonyl Iron – A fine, micron-sized, elemental iron powder (i.e., Fe) that is formed from the thermal decomposition of iron pentacarbonyl. It is particularly useful for MR fluids because of its particles’ small size and relatively uniform and spherical shape.

Center-of-Gravity System – An equipment installation wherein the center of gravity of the equipment coincides with the elastic center of the isolation system.

Coil Assembly – The non-rotating portion of the Balancing System that provides power to the Balancer Assembly and provides RPM and counterweight rotor position signals to the Controller via the Tachometer Assembly.

Compression – When specified as a direction for loading - a deformation caused by squeezing the layers of an object in a direction perpendicular to the layers.

Controller – The part of the Balancing System that processes signals from the Accelerometer(s) and Coil Assembly(ies). The controller sends command signals to the Coil Assembly via the Amplifier for the balancing function. Depending on the system configuration the Amplifier may or may not be an integral part of the Controller.

Coupled Two Plane – A Balancing System with two Balancer Assemblies for which excitation in one plane affects the second plane.

Damping (c) – The “mechanism” in an isolation system which dissipates energy. This mechanism controls resonant amplification (transmissibility).

Decibel (db) – A dimensionless expression of the ratio of two values of some variable in a vibratory system. For example, in random vibration the ratio of the power spectral density at two frequencies is given as:

db = -10Log₁₀ S_f1/S_f2

Deflection – The movement of some component due to the imposition of a force. In vibratory systems, deflection may be due to static or dynamic forces.

Degree-of-Freedom – The expression of the amount of freedom a system has to move within the constraints of its application. Typical vibratory systems may move in six degrees of freedom – three translational and three rotational modes (motion along three mutually perpendicular axes and about those three axes).

De-Roping – Removal of the adhesive squeeze-out. It is easiest to de-rope when the adhesive is still warm or wet when curing. Be aware that when de-roping soft or wet adhesive you can pull adhesive out of the bond joint and reduce the strength of the assembly.

Direct-Shear Mode (Also Called Shear Mode) – A method of damping by MR fluid in which parallel surfaces are shifted by an external force (but remain parallel). This external force is resisted by MR fluid, whose rheology is changed by the application of a magnetic field. Direct-shear-mode MR devices are suitable for clutches and brakes, chucking/locking devices, dampers, breakaway devices and structural composites.

Disturbing Frequency (f_d) – The number of oscillations per unit time of an external force or displacement applied to a vibrating system f_d = disturbing frequency.

DOCs (Direct Operating Costs) – The costs to operate an aircraft, including spare parts and maintenance.

Durometer (Hardness) – An arbitrary numerical value which measures the resistance to the penetration of the durometer meter indenter point; value may be taken immediately or after a very short specified time.

Dynamic Matching – The selection of isolators whose dynamic characteristics (stiffness and damping) are very close to each other for use as a set on a given piece of equipment. Such a selection process is recommended for isolators which are to be used on motion sensitive equipment such as guidance systems, radars and optical units.

Dynamic Disturbance – The dynamic forces acting on the body in a vibratory system. These forces may be the results of sinusoidal vibration, random vibration of shock, for example.

Elastomer – A generic term used to include all types of “rubber” - natural or synthetic. Many vibration isolators are manufactured using some type of elastomer. The type depends on the environment in which the isolator is to be used.

Ferrofluid – A liquid composed of nanometer-scale magnetic particles suspended in a carrier fluid that responds to a body-force which is proportional to the gradient of the applied magnetic field. In other words, ferrofluids do not develop a yield strength such as MR fluids. Rather, they remain liquid but experience a force that moves them toward regions of higher magnetic field strength.

Finite Element Method Magnetics (FEMM) – FEMM is a suite of software programs for solving static and low frequency problems in magnetics. It is available at the following Web site: http://femm.berlios.de/Archives/readme.htm.

Fixture Time – The time required in the fixture for the adhesive to develop enough strength that the part may be removed without disrupting the bond joint.

Fragility – The amount of vibration or shock which a piece of equipment can take without malfunctioning or breaking. In isolation systems, this is a statement of the amount of dynamic excitation which the isolator can transmit to the isolated equipment.

Free Deflection – The amount of space an isolated unit has in which it can move without interfering with surrounding equipment or structure. This is sometimes called “sway space.”

“G” Level – An expression of the vibration or shock acceleration level being imposed on a piece of equipment as a dimensionless factor times the acceleration due to gravity.

Handling Strength – The minimum bond strength necessary for assembly support without disruption of the adhesive joint. Depending on the application, generally 20 to 50 PSI shear strength.

Handling Time – The time required to achieve adequate handling strength.

HMI (Human Machine Interface) – The part of the Balancing System that houses the display and keypad for operator communication with the Controller. Depending on the system configuration the HMI may or may not be an integral part of the Controller.

Hysteresis – In reference to MR, hysteresis is the phenomenon in which the magnetic flux density does not return to zero when the external magnetic field is reduced to zero.

Inductance – Two times energy stored in the magnetic field, divided by the square of the applied current. High inductance means that a desired magnetic field can be achieved with a low current, though at the expense of slow response.

Isoelastic – A word meaning that an isolator, or isolation system, exhibits the same stiffness characteristics in all directions.

Isolation – The protection of equipment from vibration and/or shock. The degree (or percentage) of isolation necessary is a function of the fragility of the equipment.

Linear (Properties) – A description of the characteristics of an isolation system which assumes that there is no variation with deflection, temperature, vibration level, etc. This is a simplifying assumption which is useful for first approximations but which must be treated carefully when dealing with critical isolation systems.

Load Deflection Curve – The measured and recorded displacement of a mounting plotted versus and applied load.

Loss Factor – A property of an elastomer which is a measure of the amount of damping in the elastomer. The higher the loss factor, the higher the damping. Loss factor is typically given the Greek symbol “η”. An approximation may be made that loss factor is equal to the inverse of the resonant transmissibility of a vibratory system. The loss factor of an elastomer is sensitive to the loading and ambient conditions being imposed on the system.

Magnelok – A LORD magnetically responsive technology that does not use magneto-rheological (MR) fluid. Although the control envelope of Magnelok devices is smaller than that of MR-fluid devices, Magnelok devices enable a true force decoupling in the off-state.

Magnetic Field Strength (Symbol H) – Magnetic force intensity. Normal units of magnetic field intensity are amps/meter.

Magnetic Flux – A measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field.

Magnetic Flux Density (Symbol B) – The amount of magnetic flux through a unit area, measured perpendicular to the direction of the magnetic flux radiating from the poles of a magnet. Normal units are Tesla or Gauss, which is equal to 0.0001*Tesla.

Magnetic Permeability – The ability of a substance to become highly magnetized in relatively weak magnetic fields. It is the ratio B/H. Magnetic permeability is normally reported as the relative magnetic permeability, B/H/mu0.

Magnetic Saturation – The condition in which all of the magnetizable dipoles in the material have been aligned, and therefore no additional alignment can occur. The magnetic flux density will increase but at a rate that is the same as it would increase in a vacuum.

Magneto-Rheological – Related to the deformation and flow of materials by the application of a magnetic field.

Magneto-Rheological Fluid – A suspension of micron-sized, magnetizable particles in a carrier fluid. In the absence of an applied magnetic field, a magneto-rheological fluid is a free-flowing liquid having a consistency similar to that of motor oil. However, when a magnetic field is applied, the fluid’s rheology changes, virtually instantly, to a consistency like that of peanut butter.

Modulus – A property of elastomers (analogous to the same property of metals) which is the ratio of stress to strain in the elastomer at some loading condition. Unlike the modulus of metals, the modulus of elastomers is non-linear over a range of loading and ambient conditions. This fact makes the understanding of elastomers and their properties important in the understanding of the performance of elastomeric vibration and shock isolators.

MR – See “magneto-rheological.”

Natural Frequency (f_n) – that frequency (expressed as “Hertz” or “cycles per second”) at which a structure, or combination of structures, will oscillate if disturbed by some force (usually dynamic) and allowed to come to rest without any further outside influence. Vibratory systems have a number of natural frequencies depending on the direction of the force and the physical characteristics of the isolated equipment. The relationship of the system natural frequency to the frequency of the vibration or shock determines, in part, the amount of isolation (protection) which may be attained.

Octave – A doubling of frequency. This word is used in various expressions dealing with vibration isolation.

Off-State Force (or Torque) – In an MR device, the minimum force (torque) present in the absence of an applied magnetic field.

On-State Force (or Torque) – In an MR device, the resistive force or torque present when a magnetic field is applied.

Open/Work Time – The interval of time after mixing a two-component adhesive or applying a single-component adhesive to apply, mate, and position parts prior to commencement of cure. This allows the adhesive to satisfactorily wet the surfaces being bonded.

Power Spectral Density – An expression of the level of random vibration being experienced by the equipment to be isolated. The units of power spectral density are “g²/Hz” and the Typical symbol if “S_f”.

Process Time – The time from when the adhesive is applied to the part to when the parts are mated and clamped together. Adhesive selection is based on process time requirements. The adhesive work time must accommodate the assembly process time.

Random Vibration – Non-cyclic, non-sinusoidal vibration characterized by the excitation of a broad band of frequencies at random levels simultaneously. Typically, many applications of equipment in the field of Military Electronics are exposed to random vibration.

Read Thru – The visual detection of the adhesive bond on the aesthetic finish surface of the assembly. Some contributing factors to read-thru are stress, heat, excessive adhesive squeeze-out and bond thickness.

Resilience – The ability of a system to return to its initial position after being exposed to some external loading. More specifically, the ability of an isolator to completely return the energy imposed on it during vibration or shock. Typically, highly damped elastomers have low resilience while low-damped elastomers have good resilience.

Resonance – Another expression for natural frequency. A vibratory system is said to be operating in resonance when the frequency of the disturbance (vibration or shock) is coincident with the system natural frequency.

Resonant Dwell – A test in which the equipment is exposed to a long term vibration at its resonant frequency. This test was used as an accelerated fatigue test for sinusoidal vibration conditions. In recent times, sinusoidal testing is being replaced by random vibration testing and resonant dwell tests are becoming less common.

Returnability – The ability of a system, or isolator, to resume its original position after removal of all outside forces. This term is sometimes used interchangeably with resilience.

Rheology – The study or phenomena of the deformation and flow of matter in response to a force or pressure.

Roll-Off Rate – The steepness of the transmissibility curve being recorded during a vibration test, after the system natural frequency has been passed. This term is also used to describe the slope of a random vibration curve. The units are typically “db/octave.”

Rotor Assembly – The counterweight rotor including the balance weights and permanent magnets.

Rotary Inertia – The tendency of a rotating body to maintain its state of uniform rotational motion. Its angular momentum would remain constant unless an external torque were applied.

Set – Is the amount of deformation never recovered after removal of a load. It may be in shear or compression.

Shear – When specified as a direction for loading - a deformation caused by sliding layers of an object past each other in a direction parallel to the layers.

Shear Mode (Also Called Direct-Shear Mode) – A method of damping by MR fluid in which parallel surfaces remain parallel but are shifted by an external force. This external force is resisted by MR fluid, whose rheology is changed by the application of a magnetic field. Shear-mode MR devices are suitable for clutches and brakes, chucking/locking devices, dampers, breakaway devices and structural composites.

Shear Rate – For MR devices in valve mode, shear rate is not constant across the gap. Peak shear rate is approximately equal to the average flow velocity divided by one fourth of the gap width. For MR devices in direct-shear mode, shear rate is the ratio of the shear velocity divided by the gap width.

Shear-Thinning – The characteristic of fluids having a lower apparent viscosity at higher shear rates. By contrast, Newtonian fluids have a constant viscosity at all shear rates and include many of the most common fluids, such as water, corn syrup, glycerine and oils. Household examples of shear-thinning fluids are hair-styling gel and latex paint; neither pours easily off the fingers (a low-shear application), but each has much less resistance when rubbed between the fingers (a high-shear application). A shear-thinning formulation of MR fluid has a high apparent viscosity at very low shear rates, thereby mitigating gravitational settling.

Shelf-Life – The maximum allowable time the adhesive components should be stored at a given temperature before use.

Shock Pulse – A shock pulse is a transmission of kinetic energy to a system, which takes place in a relatively short length of time compared to the natural decay of the oscillatory motion. Shock pulses are usually displayed as plots of acceleration vs. period of time.

Spring Rate – Is the force required to induce a unit deflection of spring. A steel spring has a very linear relationship between force and deflection. Elastomeric springs may or may not be linear depending on the amount of deflection due to the load.

Static Deflection (d_s) – The deflection of the isolator under the static of dead weight load of the mounted equipment.

Substrate – The surfaces being bonded.

Tachometer Assembly – The part of the Balancing System that houses tachometer sensors to provide RPM and counterweight rotor position signals to the controller. Depending on the system configuration the Tachometer Assembly may or may not be integral with the Coil Assembly.

Taper Lock Assembly – Mounting interface between the balancer assembly and the fan shaft. By tightening bolts in the assembly, the balancer assembly is locked to the fan shaft.

TFD – See LORD TFD^® Steer-By-Wire Device

LORD TFD^® Steer-By-Wire Device – A device that returns haptic “feel” to the user in control-by-wire systems. A tactile feedback device can be programmed to emulate precisely the tactile feedback of a traditional mechanical system (e.g., for a steering wheel or knob).

Thixotropic – The characteristic of certain gels, such as shear-thinning MR-fluid formulations, that causes the gels to become fluid when shaken or stirred, and return to a semisolid state upon standing. This characteristic mitigates gravitational settling.

Transmissibility (T) – Is a dimensionless unit expressing the ratio of the response vibration output to the input condition. It may be measured as motion, force, velocity or acceleration.

Valve Mode – A method of damping by MR fluid in which external motion is exerted upon, and resisted by, the MR fluid contained within a vessel. The rheology of the MR fluid is changed by the application of a magnetic field. Valve-mode MR devices are suitable for hydraulic controls, servo valves, dampers and shock absorbers.

Viscosity – The property of a material to resist flow. Plastic viscosity is the slope of the shear-stress-versus-shear-rate curve. Apparent viscosity is the ratio of the shear stress to the shear rate at any given point on that curve.

Volume Fraction – Relating to MR fluid, the amount of iron per unit volume.

Yield Strength – A material’s threshold of resistance beyond which an irreversible change of shape occurs in the material as a result of an applied force.