What is an Unbalance?
To understand the importance of balancing and the purpose of balancing machines, we need to understand what is unbalanced. Unbalance exists in a rotor when its center-of-gravity is being out of alignment with the center-of-rotation. The unbalancing can be caused by several factors such as blow holes in casting, parts fitted off-center, uneven number of bolt holes, and position of bolt holes. Due to the unbalanced, the centrifugal forces will generate vibration when the rotor is rotating, it can cause wear to the bearing and create unnecessary noise. In an extreme case, we may experience the disintegration of the rotor itself. In this case, it is very important to reduce the unbalance of a rotor to an acceptable limit.
There are two types of unbalancing which are Dynamic Unbalance and Static Unbalance. Static unbalance occurs when the object is static. For instance, when a rotor is placed on smooth rails, the heavier side will turn downward. This kind of static unbalance can be corrected using one location. On the other hand, dynamic unbalance happens when the object is rotating and it can combine static unbalance with couple unbalance. The unbalancing occurs at any axial position along with the rotating object and with any magnitude. Dynamic unbalance can be corrected at two locations in the axial direction.
What is Balancing & The Balancing Machine
Balancing is the process of reducing the unbalance of the rotor to its acceptable tolerance value. To achieve balance, it is simply moving the center of gravity to the center of rotation.
A balancing machine is measuring equipment that is used for balancing rotating machines parts such as rotors for fans, electric motors, turbines, propellers, pumps, etc. It consists of two rigid pedestals, suspension, and the bearings on top supporting the mounting platform. The item to be tested is bolted to the platform and then rotated with air, belt, or end-drive at a pre-determined speed. When the item is rotating, sensors detect the vibration in the suspension to determine the amount of unbalance in the item. The information collected from the sensor can also determine how much and where to add or remove the weight in order to balance the item. The operator will then decide whether to remove or add weights on the indicated location to bring the rotor in balance. The balancing machines come with automatic or manual unbalance correction depending on the requirements on productivity and cost. Modern balancing machines are normally computerized and all the data is shown on a PC for better management and analysis.
A balanced rotor generates lower centrifugal forces and vibration levels that enable the machine to operate faster and last longer with higher reliability and productivity. It can also generate lower stresses on the bearing and maintain a long lifetime of other components.
Types of Balancing Machines
There are three types of balancing machines for different types of unbalance. Static balancing machines measure the static unbalance only; dynamic balancing machines measure the dynamic unbalance only; while the universal balancing machines can be used to measure both static and dynamic unbalance.
Dynamic Balancing Machine
Hard-Bearing Balancing Machines
Under the dynamic balancing machine, it can be differentiated into two types which are the hard-bearing balancing machine and soft bearing balancing machine. Hard-bearing balancing machines are commonly used in the manufacturing process that requires fast cycle time. They have stiff work support and rely on more sophisticated electronics to interpret the vibration. The working principle behind this type of balancing machine is that the rotor is fully constrained and then the machine measures the forces that rotor puts on the supports. In order to measure the unbalance forces from the rotating rotor, the rotor has to be spun at a fast speed to generate enough forces and be detected by the stiff suspensions.
Hard-bearing balancing machines are permanently set in place and not allowed to be moved. They also have lower sensitivity that vibration from adjacent machines or other activities can affect their balancing results. The key advantage of the hard-bearing machine is that it can provide a quick unbalance readout that is useful for high-speed production balancing.
Soft-Bearing Balancing Machines
In contrast, soft-bearing balancing machines have flexible work supports. They can be moved without affecting the calibration performance and can be placed anywhere. Their flexibility provides them natural isolation that they can still achieve fine balance levels while their nearby activities are ongoing. As the name suggests, the soft-bearing machine supports the item to be balanced on bearings that are free to move in at least one direction, either horizontally or perpendicularly to the rotor axis. When the movement of items is measured, it behaves as if suspended in the mid-air. Soft-bearing machines also provide the advantages of having higher sensitivity than hard-bearing machines at a lower balancing speed. The hard-bearing machines typically measure the force at a higher balancing speed. Soft bearing balancing machine is versatile and able to handle a wide range of rotor weights with one size of the machine.
Tips for Choosing a Balancing Machine
Selecting a balancing machine is a techno-commercial decision and we need to take into account of few considerations when choosing the right balancing machine.
Firstly, we need to know the dimensions and weight of the item to be tested, to ensure that the balancing machine is able to handle and accommodate the item. The size of the item should be physically compatible with the balancing machines. Multiple balancing machines may be required if needed to handle a wide range of items in a variety of sizes and weights. Secondly, we need to consider whether unbalance correction is required for the balancing machine or just measurement is sufficient for our jobs. If the correction is required and also the method of correction we need to decide. Additionally, the accuracy of balancing required. It is decided by the required balance tolerance that is based on the application, accepted vibration, service speed, load limit considerations, etc.
MecHero Malaysia provides balancing machines and services for a wide range of industrial applications. You can always contact us at +6011-7000 7687 or email us at my.mechero@gmail.com for an inquiry about the types of balancing machines in Malaysia. As a steel and online hardware store in Malaysia, we also supply a wide range of steel materials, general hardware, or mechanical components. Tell us your requirements now and we can advise you accordingly.
Photo credit: schenck, enset, newwayairbearings, wdbgroup, vtm