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Snap-On Techwrench™

Occasionally, we want to give ArchNews readers feedback on the tools we use. In this issue we'll discuss a new line of electronic torque wrenches offered by Snap-On, the Techwrench™ series. We purchased the TECH2, a 3/8 drive 5-100 ft-lb model, for general assembly and facilities use. As you can see in the photo below, a key feature of this transducer-based wrench is that it incorporates a standard bi-directional ratcheting head in a form-factor not far removed from a standard ratchet.

Snap-On Techwrench™

The head incorporates a 15° swivel which adds more usability than is sacrificed in accuracy due to small variation in lever-arm length. The user is alerted to the wrench reaching its set point by both an audible alarm and vibration. Units (in-lb, ft-lb, N-m) and target torque are set in a logical fashion. Power is provided by three AA cells. The retail price of the wrench is $295.00. This is far less than most existing electronic wrenches which provide ±1% accuracy at a cost of over $1,000. These wrenches are generally non-ratcheting, but can provide output capability and other data-collecting and data-processing features not available on the Techwrench™.

Techwrench™

Techwrench™ controls power, increase target, decrease target, units

Techwrench™ accuracy is stated in product literature as ±2% of reading in the clockwise direction, and ±3% counter-clockwise. Instructions included with the wrench detail accuracy fall-off below 20% of full-scale, down to ±8% at 5%-10% full-scale. While mechanical wrenches exhibit similar fall-off, some users believe that electronic wrenches have constant accuracy across their stated capacity.

One of the reasons we wanted to discuss the Techwrench™ is to look more broadly at accuracy, extending its measure to include the user's influence. If one of two similar tools with equal accuracy could be used more precisely than the other, it is effectively a more accurate tool. We wanted to see whether the user interface of the Techwrench™ can yield better installed accuracy than a traditional mechanical dial wrench. For comparison we used the 150 in-lb CDI (a division of Snap-On) dial wrench we have in our lab to handle torque range below the Techwrench™. This wrench is also rated at ±2% from 20% of capacity to full scale. Also, like the Techwrench™, it is within its factory calibration period. The retail price of a ±2% dial wrench with a 100 ft-lb capacity varies from approximately $200 to $250. For a direct comparison, the Snap-On 100 ft-lb Torqometer™ dial wrench with ±2% accuracy retails for $238.

Mechanical dial wrench tested for comparison

The test we ran involved tightening a hex-head fastener to a target torque read off the tool and comparing that value to the reading of a laboratory transducer. To remove any effect that the capacity differential between the wrenches might have on installation, we selected the target torques based on the relative lever-arm lengths of the wrenches. The ratio of the distance from the center of the handle to the drive centerline for the two wrenches was 1.65:1. The target torque values chosen for the two wrenches; 12 ft-lb for the dial wrench and 20 ft-lb for the Techwrench™ maintained that same ratio. Both values are within 20%-100% of full scale of each respective wrench. To monitor installation torque we installed a strain-gage based torque/angle transducer in-line between the wrench and the socket. The transducer output was read with a transient recorder. The transducer is accurate within ±0.25% over the range used in this test.

Test Setup - torque/angle transducer connected to transient recorder

Twelve readings were taken for each wrench. The readings were taken in a manner that we feel would represent careful production work. The results below show that user influence on the readings was minimal and that the effective accuracy of the two wrenches was approximately equal.

	Techwrench™		Dial Wrench
	Value	Deviation 1	Value	Deviation 1
Target Torque, ft-lb	20.00	-	12.00	-
Mean Reading, ft-lb	19.57	-2.2%	11.93	-0.8%
Standard Deviation	0.23	-	0.17	-
Mean -4 Std. Dev., ft-lb	18.66	-6.7%	11.24	-6.3%
Mean +4 Std. Dev., ft-lb	20.47	2.4%	12.62	5.2%
	1 deviation from target

One of the interesting elements of this test was comparing the use of a dial and a digital read-out as a user-interface. Although the Techwrench™ provides an audible and vibratory alert, a dial is a better interface than a digital display in this type of user-influenced operation as it is much more visual. In fact, the user tends to use the Techwrench's™ alerts to determine when to stop rather than the display. This reminds us of our experience several years ago when we were part of the team developing some of the first electronic hand tools (calipers, height gages, indicators, etc). The digital dial indicator we introduced was well accepted for taking linear measurements as one would from a surface plate. However, in instances where the user wanted to measure variation, like the run-out on a rotating shaft, the lack of a dial to help visualize as well as measure was not well-received, and the user stuck with the mechanical indicator. A competitor introduced a product that added a segmented display which imitated a dial. Although much more costly, it proved to be quite popular. Though a circular display wouldn't fit the Techwrench's™ package very well, adding segmented bar to the current display which progressively illuminated as the target was approached would be a nice addition.

Usability

As far as usability, the Techwrench™ behaves more like a traditional ratcheting wrench than a torque wrench. Although the body is a larger diameter than a ratchet this doesn't detract from access in most cases and results in a more comfortable handle. A couple of annoyances we came across in use were the ratchet lever, which was too small to comfortably rotate between the clockwise and counter-clockwise positions, and the auto-off function that shuts the tool off after two minutes at idle. Several times we would attempt to use the tool, not seeing that it had shut off between bolts. This was particularly common when using the wrench as a primary assembly tool, rather than as a torque wrench only. Battery life seems quite reasonable, at least in our occasional use, so a longer idle before auto-off may be a good compromise. Or, of course, the user could be more observant.

So, what is the bottom line on the Techwrench™?

Is it worth the $50 - $100 premium over a standard dial wrench? That depends on what you want to do with it. If you want a wrench only to verify torque, particularly if it is to be used by conscientious operators, the Techwrench™ really doesn't provide additional benefit. However, using the Techwrench™ to combine the function of both the ratcheting socket wrench and torque wrench is where the product really shines. Of course a dial wrench can be purchased with alerts and ratcheting adapters, but the cost then rises above the Techwrench™, the package is not as compact, and we feel relying on the alert only benefits the dial wrench in positions were it can't be read directly. We really can't comment on how well the Techwrench™ will hold up if dropped (at least not yet) or if used in an environment more challenging than a test lab. In summary, the greatest value of the Techwrench™ may be that its convenience will ensure that the fasteners which are supposed to be torqued actually will be, and that others which were previously installed by feel will get the benefit of this tool's utility.