Guidelines for selecting the right driving motor and flexible shaft features

Using Flexible Shafts in Industrial Drain Cleaning is Ideal for

Tight Twisted Environments
Guidelines for selecting the right driving motor and flexible shaft features
By David W. Neville, and Rishit Arora, S.S. White Technologies, Inc.
Industrial drain cleaning machines that use flexible shafts have several clear advantages over other available alternatives. They are smaller, lighter and more flexible; easier to clean; and enable the use of a camera in drain cleaning. They are also safer to operate and provide equipment life cycles up to three times longer than traditional power rodding machines. Follow these guidelines for selecting the right driving motor and flexible shaft features.
Flexible shafts in drain cleaning machines
Flexible shafts used in industrial drain cleaning systems feature a flexible metal cable that threads into drain systems and around bends and joints. A cleaning head with very sharp teeth, sized to the diameter of the pipe being treated, is installed at the leading end of the cable. An electric motor rotates the cable, causing the sharp cleaning head to turn at a high rate of speed - obliterating pipe obstructions, shearing off protrusions, and shaving away years of sludge buildup. Obstructive material blocking the pipe is shredded into small fragments that are easily flushed away by water flow introduced into the pipe.
The flexible shaft used is a precisely defined and configured nested group of springs, tightly wound so that it has torsional or rotational strength, as opposed to the tensile strength found in standard wire rope or cable. The shaft transmits torque between two points that are not linearly aligned with each other while maintaining high flexibility along its length.
The key feature that makes flexible shafts so useful is that they can bend while rotating. Transmitting rotary motion much like a solid shaft, the flexible shaft can also be routed over, under, and around obstacles that would make using a solid shaft impractical. This flexibility, combined with tensile strength, makes flexible shafts a force in the tight, twisted environments often encountered in drain cleaning.
Flexible shafts offer numerous advantages over traditional drain cleaning cables
Flexible shafts are as strong as other kinds of cables used in drain cleaning, but they are smaller and lighter, as well as more flexible. Their low weight and size make them easier for drain cleaning machines operators to transport to the job site. The flexible shaft is designed for high fatigue life cycle, providing 1.5 to 3 times longer life cycle for equipment compared to traditional power rodding machines.
Their greater flexibility compared to traditional stiffer drain cleaning cables makes maneuvering the cables through drains easier. This is especially important for drain cleaning applications in which the clogged pipes are particularly bendy or hard to navigate. The flexible shaft is a bi-directional rotation device that can transmit torque in both clockwise and anti-clockwise directions. This is a critical feature, especially in cases when the head accessory gets jammed into debris.
Flexible shafts are also easier to clean because they are covered in a protective casing, unlike traditional drain cleaning cables that often get clogged with debris as the cable moves through the dirty drain.
Lastly, flexible shafts clean drains without water, so a camera can be fed into the drain just behind the spinning end of the cable. This allows drain cleaners to see exactly what is blocking the pipe, enabling them to adjust their cleaning method if necessary. Since the flexible shaft machine requires no high-pressure water, it is safer to operate, and there is a reduced risk of dealing with high pressure fluid and potential ruptures in the drain.
How does the flexible shaft work?
A useful analogy for explaining how the flexible shaft works is a comparison of a solid metal rod and a rubber band. The solid metal rod can transmit very high torque in a straight length but is and is difficult to bend or transmit torque in a bent condition. On the other hand, the rubber band is extremely flexible and can be bent easily but cannot transfer torque from one end to another. A flexible shaft has both torsion and flexibility properties, which is why it can go through tight drain bends while transmitting torque and rotary motion to chain knockers.
For example, a flexible shaft with a .37-inch diameter fitted with chain knockers can clear up to 4-inch inch diameter drains. The ratio of the diameter of the drain to be cleaned to the diameter of flexible shaft required is of the magnitude of 10, hence delivering high power transmission density.
Keep in mind the limitations of flexible shafts for drain cleaning
There are two ways energy can be lost when using a flexible shaft. The first is loss between the shaft and the surrounding casing. This type of energy or torque loss is constant and is independent of the shaft trajectory. These external losses increase with length in a linear fashion.
The second loss type results from internal loss at the bends due to the layer on layer construction used for flexible shafts. There is a reaction to adjacent layers at bends during operation, leading to friction and energy loss. This can be significantly more than external losses at a smaller bend radius. In other words, internal losses are inversely proportional to bending radius.
The total loss (TE) is defined as the sum of internal and external losses. This can be expressed as
TE = Torque x rotational speed (rad/sec)
Rotational speed at the input is the same as rotational speed at the output in the flexible shaft's steady state. Assuming steady state and constant revolutions per minute (RPM) at the input and output end:
TE Input - TE output = Total loses in flexible shaft
And
(Torque in - torque out) x rotational speed (rad/sec) = Total losses in flexible shaft
This means that all the losses in a flexible shaft result in torque loss along the length. Torque at output is a function of total losses (at the bend and along the length) and input torque.
How to select the driving motor to ensure successful operation
Selecting the right driving motor is critical to achieving successful operation of a flexible shaft for drain cleaning. Collecting information on the following factors should help select the proper input motor for any given application:
• Torque required at the output end. This will depend on the type of debris in drain line. (T.out)
• Total length and trajectory of drain pipe that needs to be cleared. (L (length) and N (trajectory bends))
• RPM required to produce enough centrifugal force on chain knockers to clear debris by centrifugal push of the chain. (ω)
As derived from the equations shown above, T.in = f (T.out, ω, N, L). Hence, T.in is calculated at a particular ω. This will further provide energy required at the input end = T.in x ω.
It can be assumed that the motor for T.in will be consistent for different applications, assuming the unit is manufactured as one package. Therefore, a motor's effectiveness in drain cleaning operation can be determined based on its shaft construction, diameter, length, and actuation motor rating.
Selecting the right flexible shaft
Consider these factors when selecting the flexible drive shaft.
• Diameter of drain
• Total number of bends in the drain (for the length of the drain to be cleaned)
• Type of debris (soft or hard obstructions)
As noted earlier, the length and diameter limit are determined by torque requirements at the chain knocker or receiving end of the shaft. Debris to be cleared will dictate torque requirements. Length and diameter are determined by drain design, which is constant for specific applications. A minor increase in shaft diameter provides exponential increase in torque transfer capability, if the motor or actuation device can provide end torque requirements in addition to potential losses at motor end of flex shaft.
While knowing the specific drain blockage/obstruction and its distance from the opening can help determine the required shaft diameter and length, most commercial application packages come in standard units for commonly used lengths. Chain knockers, designed for different drain line diameters, are provided as replaceable accessories.
Pictured is Michigan-based drain cleaning contractor Clog Squad's Clog Dog flexible shaft drain cleaning machine. The company had been searching for a lighter, safer and easier way to clean drains, and found the use of flexible shafts with the Clog Dog has made the job cleaner, safer, and more effective.
To sum it up flexible shafts are a light, powerful, clean, and effective new method to clean drains that can serve as an excellent option in all kinds of industrial drain cleaning applications. Using the right actuation motor and head accessories will help achieve the best flexible shaft arrangement for most drain cleaning systems.