Your motor needs to drive a large fan 15 ft away. The connecting shaft will be heavy but there is no place to mount its support bearings. What do you do?

A floating shaft assembly may be the answer. Commonly called a floating shaft coupling, this assembly consists of a floating shaft (one with no support bearings) that is fitted with flexible couplings on the ends. Like the couplings themselves, this three-part assembly transmits torque and accommodates misalignment between connected shafts. But the floating shaft coupling assembly specializes in connecting a driving motor or engine with a driven machine when the two are spaced far apart, Figure 1.

Floating shafts can be used with conventional flexible couplings such as metallic disc, gear, and elastomeric types, as well as universal joints. Couplings with metallic flexible elements typically have enough lateral stiffness to support the floating shaft weight. However, elastomeric couplings may require special bushings to support the weight.

Configurations

Floating shaft couplings are applied in both full-floating and semi-floating configurations, Figure 2. In a full-floating arrangement (top), a shaft with flexible couplings on both ends connects the driving and driven machines. The shaft has no support bearings, hence the term “floating.” Instead, integral shaft support bearings in the two machines provide support for the floating shaft between them. A semi-floating arrangement (center) needs only one flexible coupling. This coupling connects one end of the shaft to either a driving or driven machine and is supported on that end by the machine’s bearings. The other end of the shaft is supported by a bearing located close to, for example, a sprocket or pulley. Various combinations of semi-floating and fullfloating shafts (bottom) are also used to cover long spans.

From fans to pumps

Floating shaft couplings, particularly tubular versions, can provide a cost-effective, lighter-weight option to machined steel solid spacers and jackshafts, which have intermediate support bearings.

These couplings are used extensively as cooling tower drive shafts, Figure 1, in power utilities and petrochemical processing plants. Also found in multi-color printing processes, a floating shaft coupling connects the shaft of each roll to a gearbox that drives the roll. Waste water pumping applications often use a floating shaft coupling to connect a lift pump below ground, Figure 3, to a motor located at the surface. In these applications, the pump is commonly 20 to 40 ft below the motor, but this span can range from 10 to 95 ft.

Other applications include fan drives that are used in mining for ventilation, and in wind tunnels for automotive and aircraft product development. In the latter case, a motor would disrupt aerodynamic studies if it were placed inside the wind tunnel. But a floating shaft attachment lets the motor be located outside the tunnel. Finally, paper converting and steel mill equipment also use floating shaft couplings.

Composite shafts

Since 1988, shafts made from composite materials have provided a viable option to metal shafts — solid steel and steel tubing — especially for applications where:

• The span is very long, generally 8 to 20 ft.
• Weight (of the shaft) or vibration is an issue.
• The atmosphere is corrosive.

Composite shafts are made by winding carbon or glass fibers (to which an epoxy has been applied) on a mandrel, then oven-curing the epoxy. This process creates a tubular shaft that has about 20% of the weight of steel and half the lateral stiffness. The higher stiffness-to-weight ratio of the composite tubular shaft allows spans up to 35% longer than those achieved with similar size steel tubing.

To select between steel and composite materials for different shaft lengths, use the following guidelines:

• Under 50 in. long — consider steel first.
• 50 to 100 in. — depending on requirements and conditions, choose between steel and composite shafts.
• Over 100 in. — composite material is the predominant choice unless there is an overriding factor.

Composite shafts can be customized to suit different applications. A user’s requirement for stiffness, torque capacity, or damping of torsional vibration can be met by adjusting the material properties and winding angles. For example, material options include carbon, glass, or a combination of 50% carbon and 50% glass. Composite materials also possess good chemical resistance compared to steel, which must be coated (yet it is still subject to oxidation when the coating breaks down) or made stainless (which may be cost-prohibitive).

Cooling tower designers embraced the use of composite shafts quickly because the long coupling spans and high sensitivity to unbalance place a large premium on weight reduction. In addition, the cooling tower industry was already accustomed to using composite materials in other equipment components.

The waste water treatment industry — where long unsupported spans reduce cost and simplify design by reducing the number of intermediate bearings — is also a major user.

An example of composite shaft weight savings is a cooling tower application where the motor runs at 1,800 rpm. The original shaft consisted of steel tubing with 10 3/4-in. OD and 3/16-in. wall thickness, and weighing 400 lb. A carbon composite shaft that replaced the steel tubing has 6 1/4-in. OD and 1/8-in. wall thickness, and weighs only 100 lb.

One new application involves automotive test stands, where the shafts are only 70 to 80 in. long, but they turn at speeds up to 8,000 rpm.

Composite shafts are not a good option for printing applications where maintaining registration is important. Here, there must be no torsional windup, which limits you to using a steel drive shaft.

Critical speed

The key to choosing between composite and steel shafts is to determine the safe lateral critical speed, or “whipping speed,” of the shaft, then make sure you can stay safely below it. At critical speed, a shaft tries to rotate about its true center of mass (or balance) rather than the center of rotation. Then whipping, or the “jump rope” effect, occurs because the bearings restrain the ends of the shaft. Where oversize steel tubing or intermediate support bearings are required to stay within safe limits, composite tubing may be a good option.

You can calculate the lateral critical speed of a tubular shaft from:

CS = (C/L2)(OD2 + ID2)1/2

where:

C = A material constant. For steel, C = 4.76 x 106. Values for composite materials typically range from 3.86 x 106 to 7.45 x 106, depending on the material. These values may also vary with the winding angle and application. Therefore, consult the shaft manufacturer for recommended values.

OD = Outside diameter, in.
ID = Inside diameter, in.

L = Effective length, in. This is normally chosen as the distance between the ends of the driving shaft and driven shaft, but it is better to use the bearing-to-bearing distance.

This formula can be used for plain tubular shaft assemblies that have no slip splines, flanges, or other heavy components located more than 4 to 6 in. from the ends.

A safe running speed for couplings with steel tubing is 67 to 75% of critical speed, depending on the application and configuration. Composite tubes are commonly run at 75% of critical speed.

The lateral critical speed must also be kept a safe margin away from other exciting frequencies in the machinery train. For example, pump and fan drives may vibrate every time a vane or blade rotates past a support column. Where these vanepassing or blade-passing frequencies are involved, keep the critical speed above or below the passing frequency by at least 15%.

John Malik is the engineering manager of the T.B. Wood’s Sons Co. plant, San Marcos, Texas.