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How can I back up my automated power tools?

 
   
 

 

The line is running fine, with all of the automated operations functioning smoothly. Productivity is high, quality is high, and some of the variable costs and direct labor costs have been reduced. The single- multi-spindle nut runners are performing their jobs pretty reliably. Only rare instances of customer complaints over the few months since the automation has been installed, and it even looks like the economy will pick up. Things are looking good!

Things are looking good! They looked good to the passengers on the Titanic. They looked good to the shareholders of Enron. They looked good to - okay, you get it. Sometimes things look very good just before disaster strikes. The question is not whether things look good, but whether they are good. And that's a question of data acquisition and analysis.

Or more simply put, are things in good shape or is the acquisition and reporting of data inadequate to warn of potential problems? Is the radar reporting a full 360° sweep or is there a gap in the coverage?

Time to go back out into the plant and take another look. This look is not to see what the automation has done, but for where the reporting system has gaps - where the radar used by our systems is not functioning.

Gazing out over the plant floor we see a fairly typical modern assembly operation, much like the one to the right. In its' most basic form we have a primary line that moves the product from beginning to end. Along the way, individual components and subassemblies are added to the product. There are subassembly lines where the subassemblies move from their beginnings as a stack of components to a finished subassembly prior to being assembled into the product at appropriate stations. All in all, Henry Ford would recognize the operation and Ely Whitney would not feel entirely out of place.

At each of a number of stations, the operation uses threaded fasteners. This is quite common since they are still one of the best fastening methods ever devised. Properly installed on a well-designed product, they can provide rigidity of structure, rapid assembly, security of connection, and the ability to effectively disassemble and properly reassemble the product for maintenance during service. That's why they are designed into the product and process, and why so many are purchased each year.
 
To assure the fasteners are properly torqued, automated torque application devices have been installed on the line. These automated tools provide rapid and efficient torque application, and many (if not all) of them communicate with the line control system. This provides assurance that each operation has been properly performed before the line controls advance the product to the next station.

Is this occurring at every location where the fasteners are being installed? Are we getting data on all of the important or even the critical torques? Or are there gaps in the radar coverage?

Hold it. That operator is using an automated tool on one fastener, but to fasten the second component at that station a torque wrench is being used. And at the next station the hydraulic lines are being torqued, but with a torque wrench. And none of them are connected to the line control system.

Let's draw a diagram of the line showing the locations of our radar coverage - where data is obtained and used to control the system - and where there is no radar coverage - where the gaps are.

Hmmm. Looks like there are gaps in the control system. Quite a few of them in fact. What is communicated to the system where the torque wrenches are used at that station? And how?
I see. The operator is pushing a button at the first station when the operation is complete, and at the second station the operator is pushing the product down the rollers past a proximity switch. In each case, there is human intervention in the reporting process between the torque application and the actions resulting from its' completion. Is this a cause for concern?

Having made a map of our torque data acquisition, we can now look at and evaluate the risks associated with the gaps in our coverage. An initial list might look like the one below.

  • Missed fastener normally tightened by automated torque tool when that tool is down for scheduled or unscheduled maintenance.
  • Under-torqued fastener normally tightened by automated torque tool when that tool is down for scheduled or unscheduled maintenance.
  • Over-torqued fastener normally tightened by automated torque tool when that tool is down for scheduled or unscheduled maintenance.
  • Fastener normally tightened by operator with a torque wrench, over-torqued.
  • Fastener normally tightened by operator with a torque wrench, under-torqued.
  • Fastener normally tightened by operator with a torque wrench, completely missed.
  • New operator on fastener normally tightened by operator with a torque wrench, under-torqued, over-torqued, or completely missed.

Missed opportunity for greater line speed and reduced operator effort via automated communications to line controls.And where are the backup tools? What happens when the automated tools are taken off-line for service - planned or otherwise? Backup automated tools for the whole plant? Did we start printing money in the basement?

There is obviously one over-riding question that arises from the analysis: "How can these risks be reduced on a cost-effective basis?"
Historically there have been just two options available:

1. Backup multi-spindle tools for all locations where they are used on the line – which incurs an incredible level of expense for the reduction in risk.
2. Accept a high level of risk to quality, productivity, and profits.

Obviously what is needed is a cost-effective product that closes the gap. That means the solution must offer the following advantages, and do so at a fraction of the cost of backup tools.

1. Accurate application of torque must occur.
2. Proof of the accurate application of torque must be demonstrated.
3. Proof of the accurate application of torque to the correct number of fasteners in the assembly must be obtained.
4. Both of the above must be communicated to the line personnel in real time.
5. The completion of the operation must be automatically communicated to the line controls in real time.

S/R now offers not one solution to this dilemma, but four ways to bridge the gap – with surprisingly low cost and very high effectiveness!

These systems have at their heart the S/R Switch Wrench and S/R FM Switch Wrench – two tools that each perform the same functions, but communicate using different means.

The Switch Wrench is a preset, interchangeable-head, clicker-type torque wrench designed specifically for use on production lines. This basic design has been one of the most popular manual torque tools in assembly plants for decades. Its’ combination of accuracy, repeatability, durability, versatility, ease of setting, lightweight, and slender body have made it popular in a variety of industries.

As a preset torque wrench with +/- 4% I.V. accuracy, the tool itself is quite capable of delivering very good accuracy on a repeated basis. To match or exceed the accuracy required for the process however, it must be used properly - every time.

To assure proper use it is necessary to examine what error modes are likely to occur and to prevent them. There are two common errors in using all clicker-type torque wrenches; too brief an application of force (no click or "jerking" the wrench, and too long or too much application of force (pulling past the click).

The S/R families of Switch Wrenches have tied the internal torque control mechanism to an electric switch. A microswitch is normally open, and closes and when the wrench "clicks". Because the microswitch is activated at the instant of the "click", and opens when pressure is released and the wrench resets, the length of time the wrench is in the "clicked" position can be measured by measuring the duration of current flow through the circuit.

There is an optimum time of click duration for the proper tightening of every joint, and a tolerance window of acceptable performance. While the time and tolerance will be different for different joint conditions, there will be a specification that results in proper tightening.

Applying that time specification to a measurement of the time during which current flows through the circuit provides a simple Go/No Go (Accept/Reject) measurement of how well the wrench was used.

The questions remaining are:

  1. How will we get the information of when the microswitch has closed and opened to a measurement and comparison device?
  2. What will measure the time the microswitch is closed and compare it to the standard for an acceptable wrench use?
  3. What will count the number of acceptable cycles (clicks) and communicate them to the operator and line controls?

We have two versions of the Switch Wrench; hardwire and FM. The hardwired version uses a slender cable that can carry up to 24 VDC at X amps, and can be used either through the PTV or by wiring directly to a PLC. The FM version uses an FM radio signal [900 MHz band] to communicate with either the PTV-FM or the TCV.

In each version, the signal transmission or current flow begins at the start of the wrench cycle [click] and stops when the pressure releases.Those who prefer hardwire communications are split; some users wish to have the wrench cycle duration and count measured and feedback created by a device dedicated to that function, and choose to use the PTV. Others prefer to output directly to the PLC’s used for line control.
The PTV is a device dedicated to efficiently performing several tasks:

  • Storing the specification for wrench cycle duration.
  • Receiving the output of the Switch Wrench.
  • Comparing the signal from the Switch Wrench to the specification.
  • Informing the operator of the acceptability [or lack thereof] of the cycle for that fastener.
  • Storing the specification for the number of fasteners on the assembly.
  • Counting the number of acceptable cycles of the Switch Wrench on the assembly.
  • Informing the operator and the line controls when the proper number of wrench cycles matches the number of fasteners on the assembly.
  • Resetting for the next assembly.

The functions are complex, but they make operation simplicity itself! To show how simple it is, let’s assume we are going to be tightening five wheel nuts on a car line. For purposes of this walk-through, we’ll assume that the wrench should click for between 0.3 and 0.9 seconds. We program the 0.3 seconds as the minimum time, the 0.9 seconds as the maximum time, 5 for the number of fasteners on the assembly, and the count direction to be upwards [0 , 1, 2, 3, 4, 5]. That process takes about two minutes, and need only be performed initially and when the number of fasteners or the joint changes – like when the next model year comes around and the new car uses a four-bolt pattern.


 

Since we will be using a socket and will have several rotations of the nut before reaching torque, we install the S/R Interchangeable Ratchet Head on the Switch Wrench, and the socket on the ratchet. That’s another 30 seconds.

We preset the torque on the wrench by testing and adjusting it on a torque tester with the S/R CART Tool, and lock in the torque setting so it remains the same and can’t be mistakenly changed. This occupies another 3 minutes.

We plug the Switch Wrench into the PTV, and we are ready to start the line. If you add the times together, it probably took longer to fill out the receiving paperwork than it did to get the unit ready for use!

The first wheel comes to the station and the operator uses the Switch Wrench to tighten the first fastener. Since he did it right, the green "ACCEPT" LED on the front of the PTV lights up, and the count of fasteners tightened on the assembly changes from "0" to "1" – and is displayed on the big, bright, LED Display. The operator continues tightening the fasteners, and the count continues to rise as each is properly tightened. When the fifth one is properly tightened, the PTV issues a pair of beeps, sends a signal to the PLC to advance the line, and resets the count to zero.

It’s that simple!

What if the operator improperly uses the tool and the fastener is either under- or over-torqued? If the wrench is "jerked" so there is not sufficient torque on the nut, there is no response from the PTV – the green light remains unlit and the count does not advance. If the operator pulls past the click and over-torques the fastener, the red "REJECT" LED lights up and the PTV issues a single beep. The "REJECT" LED will remain lit until the operator loosens the fastener and properly torques it with the Switch Wrench, at which point it will get an "ACCEPT" and the count of properly-torqued fasteners will advance.

For our demonstration, let’s assume that we were always starting with the nut closest in a clockwise direction to the valve stem. Even if the operator was interrupted part way through the assembly – called away from the station for some reason. Upon return, all he’d need to do is glance at the number on the front of the PTV, and he’d be able to tell exactly where he left off before the interruption. No missed fasteners!

There is an additional advantage that lurks in the PTV; training of new personnel can be performed while still assuring torque compliance. Suppose we need to put our new operator, Jan, on this station. She has no experience with torque wrenches, and it’s pretty much a given that she will occasionally pull the wrench too little or too much. The PTV will inform her with every use whether the wrench was used properly or not, thereby reinforcing good tool use and alerting her to the need for action whenever an error is made – as well as protecting the customer from the results of error! After investing a few minutes in OJT, demonstrating proper tool use and the actions to be taken when the PTV either doesn’t advance or beeps, and she’s ready to go to work. The PTV and Switch Wrench will assure that there are five proper tightenings on every assembly!

For those preferring to use the Switch Wrench directly with a PLC, the functions of the PTV must be programmed into the PLC or foregone. Some users, particularly those with advanced PLC programming skills, do prefer this structure.

The PTV-FM and FM Switch Wrench perform the exact same functions in the same manner as the PTV, but use FM communications instead of having a wire between the PTV and the tool. This accommodates those who need a longer distance between the PTV and the tool, as well as those who lack the space for (or taste for) wired units.

The TCV also uses the FM Switch Wrench instead of the hardwired version.

The TCV might be considered as the "PTV Light". It performs the same wrench use time comparison as the PTV and PTV-FM, but does not have the batch counting and large display on the front of the unit. It is also more compact, allowing installation in tighter quarters, and is programmed via a PC and supplied software.Let’s look at our map of coverage again. With PTV's , TCV's, and Switch Wrenches augmenting and backing up our power tools, we have achieved the goal of eliminating the gaps and their related hazards. In addition, we have provided cost-effective backup for out tools - without the high expense of duplicating the tools!

NOW we have 360° radar coverage!

For a FREE, In-Plant Demonstration, just contact us!