Torquing – The Solar Installation Mystery!

Loosely speaking, torque is a measure of the turning force on an object such as a bolt – and as Dan Shugar, CEO of Nextracker stated : “Torquing is by nature inexact because of the many factors that can affect friction – from surface texture to debris, rust and humidity”*. In this article I will try to shed some light into the mysterious world of torquing in the solar installation world. 

Correct Torque

Bolts of various sizes are used to construct solar mounting system. The required torque values are provided by the manufacturers. Pre tensioning the bolts using a calibrated wrench is the preferred installation method.

How to use a Torque Wrench

Apply pressure in a steady and smooth motion, avoid sudden, “jerking” movements. When the wrench reaches the set torque the handle will automatically click. When the wrench has clicked apply pressure one more time to insure that the wrench is clicking. You will be at the “correct” torque (see below – residual torque / ambient temperature) and can then mark the nut and thread with a colored paint marker.

Torque marks

Correct way to mark the bolt.

Torque marks

  • The purpose of marking the bolt is to see if the bolt or nut moves over time. This is a reference so that during inspections you will be able to see if there has been any movement.
  • It is important when marking the bolt that you mark the thread, nut, washer and surface of the racking material.
  • In situations where there is no nut on the bolt (Bolt goes into receiving mounting clamp hardware) as in the Schletter system, you will mark the bolt head, washer and racking surface

Torque Test

The standard procedure is that a project supervisor is checking the torque days and weeks after the initial tensioning. The outcome is usually – DIFFERENT – which leads to discussions between the involved parties – why is that the case?

Residual Torque

It is the amount of tension that remains in a joint after fastening a threaded fastener.

Many users may want to verify residual torque. By checking the torque after assembly, you not only verify adequate torque was delivered to the fastener, but may also detect missed or loose fasteners, or joint relaxation. But since the application is already seated and friction during rundown is different than the friction in a static joint, the torque reading will vary from those in the tool crib and from the dynamic values. These differences will need to be accounted for when engineering a residual torque specification.

Ambient Temperature

Most fastener materials are temperature sensitive. When a significant amount of stress is placed on a bolt and it is then exposed to a high temperature the bolt begins to relieve itself of some of the stress and ultimately, reduces the clamping force. Since the stress and the preload are related, this implies that the clamping force with which the bolt holds the joint together will be significantly reduced. Thermal Expansion is one of the most problematic temperature effects. As the temperature rises, heat causes all bolt and joint materials expand, but not all at the same rate.

This is the reason why this type of torque inspection trying to check a specific torque value is Inconclusive!

Alternative Testing

Here is a much more distinct industry approach with no room for interpretation which takes into account the circumstances of the project site:

Inspection of bolt installation, according to RCSC Specification Chapter 9.2.2, should include routine observation to ensure that the bolting crew has properly applied the calibrated wrench to the turned bolt. Depending on the project location and ambient temperature during installation, relaxation of the bolt after installation can be observed. In order to reach structural integrity and electrical bond of the connection, 50% of the installation torque is sufficient. The following test insures proper installation safety. The same calibrated torque wrench used for the installation should be used for the testing:

  1. Set calibrated torque wrench to 50% of the installation torque
  2. Attempt to loosen the bolt
  3. If the bolt does not move, the pre tensioning is sufficient and the bolt has passed

Re torquing – periodically – Why?

A significant advantage of a bolted joint over other joint types, such as welded and riveted joints, is that they are capable of being dismantled. This feature however, can cause problems if it unintentionally occurs as a result of operational conditions such as wind for example.

Such unintentional loosening, frequently called vibrational loosening is an important phenomenon and most of the racking manufacturers are referring to this in their maintenance section by advising clients to check the torque at least annually!

How do project owners feel about this? They think the installer did a bad job …. Even though the torque was checked, the bolts were marked and the quality control team did their job. How can it happen that nuts and bolts are getting loose – it can only be a mistake of the installer.

From a technical perspective I would like to offer a different reason – vibration – have you ever seen or asked for the vibration test for the fasteners used for that system – maybe you should!

Vibration test

Under a standard transverse vibration test (DIN 65151), commonly called a Junker test, prevailing torque nuts tend to self loosen initially but retain a residual amount of preload. That is, such nuts will partially come loose on test but the loosening will stop when a certain level is reached.

To me it’s not really a question whether vibration occurs on a solar rack – it’s a fact – therefore loosening of fasteners is given and the maintenance has to be done sooner or later.

Alternatives

Coming back to Dan Shugar and his company’s approach of using a kind of rivet system for their product. It has the huge advantage of being a fire-and-forget approach as you don’t have to worry about torque marks and torque values, but at the same time you will probably incur higher costs in the future while exchanging modules or when disassembling the system (if that is necessary). The final decision is up to the customer and time will tell in which direction the market is going to go.

I would like to offer another approach which is common for many other industries but has not been really introduced into the tracker or racking market – GLUE. We have seen the first examples with the First Solar modules being glued to the substructure and I strongly believe we will see more of this in the future!

Helge Biernath

*Quote from Solar builder “Solar tracking system advancements: What’s the impact over the life of a PV plant?” JULY 31, 2017BY 

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