Mis à jour : 23 avr. 2020
The importance of applying proper torque values
Drive trains being misaligned is a common phenomenon in plants around the globe. Luckily the majority of industries invested in alignment tools and training to minimize the parallel and angular offsets in machine trains. It affects the lifetime of assets, reduces power consumption and optimizes production capabilities.
It is not always easy to detect misalignment on machinery that is running. The radial forces that are transmitted from shaft to shaft are challenging to measure externally. By using vibration analysis or infrared thermography, it is possible to identify primary symptoms of misalignment such as high vibration readings in radial and axial directions or abnormal temperature gradients in machine casings. Still, without such instrumentation, it is also possible to identify secondary machine problems, which can indicate inaccurate shaft alignment.
CAUSES OF MACHINE BREAKDOWN
Loose or broken foundation bolts
Loose shim packs or dowel pins
Excessive oil leakage at bearing seals
Loose or broken coupling bolts
Some flexible coupling designs run hot when misaligned
If the coupling has elastomeric elements, look for rubber powder inside the coupling shroud
Similar pieces of equipment are vibrating less or have a longer operating life
An unusually high rate of coupling failures or wear
An excessive amount of grease or oil inside coupling guards
Shafts are breaking or cracking at or close to the inboard bearings or coupling hubs
There are many aspects to machine alignment, such as the understanding of what it means when the center lines of shafts are not colinear, the proper shimming, what is a soft foot, what kind of coupling types exist, what are the tolerances and best practices. One other major factor in the appropriate machine installation and alignment is the torque that is applied to the hold-down bolts.
THE IMPACT OF TORQUING
Hold-down bolts need to be torqued each time in the right sequence to avoid that the bolts are too tight or too loose. Otherwise, fasteners can break, or increased vibration occurs. The looseness is usually picked up by the vibration team. (1x and occasionally its harmonics as shown in image 2)
It is imperative that the clamping forces are as even as possible during the soft foot measurements but also during and after the actual alignment process.
WHAT IS THE RECOMMENDED TARGET TORQUE
The target torque depends on many factors. An OEM should have specified the values originally. If not, one needs to go by the grades of the bolts, thread type and nut factor. Each fastener grade has specific limits based on the strength of its material. The usual grades are 5 or 8.
How to identify fastener/bolt grades/thread size/nut factor?
The grade can easily be determined via the grade markings on the bolts, as shown in image 3. Grade 5 has three lines, and Grade 8 has six lines on its head. The bolt grade must be identified correctly.
Since now the grade is determined, what is the recommended target torque value?
There is another factor that plays a role in identifying the proper target torque. The thread size needs to be considered, too.
Coarse threads are those with larger pitch (fewer threads per axial distance), and fine threads are those with smaller pitch (more threads per axial distance). Coarse threads have a larger thread form relative to the bolt diameter, where fine threads have a smaller thread form relative to bolt diameter.
Lastly, the nut factor, K, sums up the combined effects of many variables affecting the difficulty of tightening the bolt, such as friction.
Some of these factors include:
The type and material of the bolt, washer and nut
The presence and nature of any plating, coating or lubrication
The pitch or angle of the bolt threads
Corrosion and wear
Often, field operations personnel will use the chart included with a torque wrench to determine the amount of torque they should apply to a bolted joint to get the right amount of tension. However, these torque charts typically are based on the assumption that K=0.2 and experiments show that this assumption often may not be accurate.
Since we have now reviewed the three factors, one can find the right torque target in published torque tables.
As an example, a 1-1/4 inch bolt grade 8 (K = 0.2) with a fine thread has a recommended torque target of 2,012 ft-lbs according to the Fastenal Tightening chart shown in image 4.
It is recommended to use a cross torquing pattern, as shown in image 5. Two to three passes are suggested.
Depending on the size of the motor and the desired target torque values, different methods can be used to induce the appropriate force.
1. Ring Spanner - Wrench
The most popular system to tighten bolts is a standard wrench or ring spanner, especially on smaller standard motors and machine trains. There is, however, no control of how tight the bolts are tightened.
2. Mechanical or digital Torque Wrench
In most instances, a socket wrench with unique internal mechanisms is recommended and used. It allows the operator to set the torque applied to the fastener so it can be matched to the specifications. There is a limit in the maximum torque that can be achieved.
3. Manual Torque Multipliers
If higher torque values are required, there is the possibility of using manual or offset multipliers in combination with a mechanical or digital torque wrench. These tools are easy to use, economical and require little training.
4. Electric/Battery/Pneumatic Torque Multipliers
An elegant method is the use of an electric or air-powered multiplier. In case of obstruction at the motor feet, which is most of the time the case, they can be used in combination with an offset multiplier due to the low profile requirements, as shown in image 11. The input of the torque multiplier is clearly defined. It is multiplied with the ratio of the offset multiplier, which allows users to set the precise and accurate torque output throughout the process with excellent repeatability and accuracy.
Fast, accurate, reliable and precise
The offset multiplier is the ultimate answer for gaining access where standard multipliers are too tall or bulky to fit the application.
5. Hydraulic Tightening of Machine Feet
Due to the low profile requirements, hydraulic torque wrenches are popular. The torque is set on the manometer of the pump and precisely defined by the torque chart that applies to the used hydraulic wrench.
Benefits and special features of hydraulic wrenches
Torque accuracy not affected by interference from lateral forces
There are many things to consider when doing an installation and alignment of drive trains, and frequently the proper torquing of the hold-down bolts and flanges of attached pipes (pipe strain) are neglected.
Identify target values if not provided by OEM.
Select the appropriate torque tool.
Apply repeatable and accurate torque values to assure precise results.
Use the Cross Torque Pattern Method to ensure even distribution.
Applying the right torque with an adequate tool is one of the many required steps to optimize the operation of machine trains!
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