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Vibration and out-of-balance equipment

ABSTRACT

When spinning equipment is out of balance. Vibration from out-of -balance rotating equipment can be frightening. The ground shakes, machines jump about, hold down bolts come loose and parts break. An unbalanced rotating body will produce forces on its bearings and transmit them throughout its structure and into the foundations.

Keywords: rotor, centrifugal forces, balance quality.

CAUSES OF OUT-OF-BALANCE

Below lists some common causes for unbalance.

Bent or bowed between support bearings

Overhung weight bends shaft under gravity

Unevenly distributed solid or liquid inside rotor

Loose parts on the rotor

Eccentrically manufactured diameters on the rotor

Misalignment of the drive train to the rotor axis

Loose drive couplings flop about

Loose tolerances between assembled parts on rotor

Shoulders on rotor manufactured out-of-square to axis

Voids or cavities within the rotor

Misalignment of bearings force shaft to bow

The drawings below provide examples of some of the problems listed in the table above.

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GEAR PUMP OPERATION & MAINTENANCE

ABSTRACT

Gear pump operation & maintenance. A gear pump uses two meshing, toothed cogs to force liquid from the inlet of the pump through to the outlet. Being a positive displacement pump there deliver very precise quantities for each revolution and this means they have good dosing characteristics regardless of their speed or the pressure into which they discharge.

Keywords: internal, external teeth, pressure relief, viscosity.

Figure No. 1 shows a simplified drawing of an external teeth gear pump on the left along with the alternate arrangement of internally pointing teeth.

pump maintenance

Figure No. 1. External and Internal Teeth Gear Pumps

GEAR PUMP DESIGN

Gear pumps use toothed gears turning inside a close tolerance housing to draw-in liquid and then squeezing it out ahead of them. Paddle steamers used the same principle of operation. These pumps are positive displacement pumps and anything drawn into them will be forced out. As a consequence they can generate very high discharge pressures. Materials of construction vary from metals of various types and hardness through to plastics of various types and hardness.

Maintaining the close tolerances between the housing and the cogs is critical to efficient operation. The clearance between the edges, of the teeth and the housing and the ends of the cogs and the back and front walls of the housing are very small. Between the teeth and housing it is in the order of 0.1 mm (0.004”) while the clearances between the front and back faces of the gears and the ends of the housing are only 0.025 mm (0.001”). The fine clearances reduce liquid re-circulation back from the high-pressure discharge to the low-pressure suction side and make these pumps one of the most efficient available.

Gear pumps usually have one shaft penetration through the housing for connection to the drive. The gear shafts on the smaller pumps can be supported in journal bearings within the ends of the housing and are lubricated by the product. On larger pumps rolling element bearings mounted in bearing housings are used. To prevent surface to surface contact of teeth the product provides the lubrication.

gear pump, pump design

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HOW FLUIDS FLOW IN PIPES (Cont.)

VISCOSITY AND DENSITY EFFECTS

Liquids do not all behave the same. Blood has different flow characteristics than water. Paint flows differently to gasoline petrol. Liquids are categorized by their behaviors when undergoing shear. Those liquids that have a constant shear rate with change of velocity (like water) are called Newtonian ( Newton first developed the mathematical explanation for the phenomenon). Those with shear rates that vary with changing velocity (like paint and blood) are Non-Newtonian. The shear rate is a measure of a fluid’s viscosity or slipperiness.

The density of a fluid affects its viscosity. Fluids with more mass per unit volume are heavier and require more energy to move them and shear less easily. A temperature rise decreases the viscosity and density of liquids.

The more viscous, or less slippery, a fluid the harder it is to get shearing between layers. The high viscosity prevents rapid velocity changes occurring between layers. The sub layer in viscous fluids is thicker than in low viscosity fluids.

VELOCITY EFFECTS

At low speeds the whole flow across a pipe is laminar and the fluid slides over itself. As the speed becomes faster eddies start to form and cross the fluid layers. A transition from laminar to turbulent flow develops. At still higher velocities the flow in the core of the pipe becomes turbulent with swirling eddies throughout. Figure 2 shows where the various flow regions occur at a tank nozzle.

The laminar sub layer is always present against the pipe wall. But as the velocity rises the energetic swirling eddies begin to impact more deeply and the sub layer begins to thin. At still higher velocities the sub layer thins further and the taller roughness peaks stick into the turbulent region. Where the sub layer covers the roughness projections the wall is considered ‘smooth’. When the wall roughness pokes out of the sub layer the wall is considered ‘rough’. This means the same wall can be both smooth and rough depending on the fluid’s velocity.

Experiments have proven the pressure loss along a pipe with laminar flow is proportional to the velocity (p µ V) where as for turbulent flow the pressure loss is proportional to the square of the velocity (p µ V²). A slower flow permits a thicker sub layer and creates a ‘smooth’ pipe wall. This minimizes the losses along the pipe. There is a very much greater loss of pressure in turbulent flow.

The pipe system designer has to strike a practical balance between increasing the pipe diameter to reduce energy loss and keeping the diameter small to lower installation costs.

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