Mechanical Seals

The majority of mixers operate in an "open" tank, or a vessel which is vented to the atmosphere. However, many mixers must operate in a "closed" tank, which requires a seal to contain the contents of the vessel. This may be required for various reason such as a pressure differential within the tank, product toxicity, flammability, environmental reasons, or in the case of side-entering or bottom-entering mixers, simply to stop the product from leaking out of the tank. Mechanical seals are used where very little or no tank emissions are allowed.

The family of shaft seals known as mechanical seals is the most advanced type of seal used in the mixing industry. They can handle the highest pressures, maintain nearly leak free operation, and require minimum maintenance if installed and operated properly.

Mechanical seals are increasing in popularity due to the growing environmental restrictions regarding leakage from process tanks.

There are hundreds of mechanical seal designs, but they all are variations of a basic layout consisting of a collar mounted on the shaft which usus springs to push a ring (which also rotates with the shaft) against another ring which is held stationary. The rings rotate against each other riding on a thin layer of lubricant, and the springs hold them so tightly together that leakage through the seal is reduced to an immeasurable amount.

The mating surfaces of the rings must be perfectly flat to seal properly, and are manufactured to tolerances measured in "light-bands". The rings must also be extremely hard to endure the pressure and wear, so they are usually mede up of ceramic, carbon, silicon carbide, tungsten carbide or similar material.

The stationary "seat" is held in place and maintains a static seal with the mounting housing using gaskets or o-rings. The rotating elements of the seal must attain a static seal with the shaft using o-rings, wedges or packing.

There is a type of mechanical seal that requires no lubricant. These "dry running" seals use a carbon ring rotating against a harder ring so the carbon ring is that sacrificial part. Any debris shed by the seal can be captured by an optional "debris well", which is flushed during a cleaning cycle. These seals are common in the pharmaceutical industry.

- Jeremy Higginson

New HYF-218 Impellers!

Sharpe Mixers now stocks 22" dia. HYF-218 impellers with both 1.25" and 1.5" bore sizes in SS316 material!

These impellers join our standard line of HYF-218 impeller sizes 7" thru 18" dia. in 0.75", 1.0", and 1.25" dia. bore sizes.

All impellers are offered in SS316 material, solid hub with set screw design as our standard. Additional coatings such as rubber or halar can be offered at an additional charge. Impellers can also be specially manufactured in special alloy materials.

How can we help your mixing needs?

Mixer School: Agitator Shaft Design

Agitator shafts are required to transmit the power from the mixer drive to the impeller(s). The shaft must handle the loads occurring including the transmission of torque, overhung moment due to hydraulic impeller forces, and thrust. The shaft must also be stiff enough to limit vibration and deflection to acceptable levels. Improperly designed, a mixer shaft may fail and cause catastrophic damage. Agitator shafts are commonly built from solid round bar, hollow shafting, or a combination of both.

Shaft strength is determined by the section modulus of the shaft.

SOLID SHAFTING has been the most common choice among other mixer manufacturers for many years because of availability, ease of construction and the fact that many impeller hubs require a key to transmit torque. These hubs slide onto the shaft and are held in place with a key and set-screw. The disadvantage of keyed one-piece hubs is that they can be difficult to service, steady bearings must be disassembled to allow clearance for impeller removal, and the impeller will usually weigh more due to the required thickness at the bolted blade attachment area. This extra weight coupled to the heavier solid shaft equates to a greater thrust load on the mixer bearings, limiting bearing life.

HOLLOW SHAFTING is Sharpe Mixers' preferred choice due to the design advantages offered, increased availability, and innovative methods of attaching impellers to the shaft without the use of key-ways, like our split hub design. Comparing a solid shaft with a hollow shaft of equal section modulus, both will transmit torque with equal stress levels, but the hollow shaft will be stiffer, or rather will deflect less under the same overhung moment. This translates to longer allowable shaft lengths at a given stress level, and less weight at a given shaft length. Less weight means less thrust transmitted to the drive bearings, and a longer expected bearing life. Less weight and higher stiffness also helps avoid critical speed, frequently a limiting factor in mixer design.

Regardless of the style of shafting used on a mixer, the important concern is that it is designed correctly for the application. Many factors regarding the tank and process enter into the design of a mixer shaft. Let us determine the best design for you today!