Frequently Asked Questions

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This column is where we address some frequently asked questions (FAQ) related to torque and torque controlled tightening.

The list will be extended continuously.

If you have a question that does not appear here, please do not hesitate to contact us directly.
Our advisor team will help you gladly.

improves the capacity of impulse wrenches

Yokota‘s patented twin-blade impulse mechanism is equipped with two blades which separate the oil cylinder into two opposing oil chambers. The oil pressure affects both blades simultaneously. This gives a steady high impulse leading to a high torque on the joint.

The impulse mechanism delivers one impulse per revolution (cf. ill. 1–8). During phases 1–3 and 5–7 the oil can flow unhindered and no oil pressure build-up occurs. The same applies to phase 4 (B). Even here no oil pressure build-up occurs because the sealing faces cut on shaft and cylinder. Only in phase 8 (A) the sealing faces are sealing and oil pressure build-up occurs. The duration of oil pressure build-up and thereby the duration of the impulse is very short.

The impulse is transferred by the blades to the main shaft and this affects an increase of torque on the joint. Between the primary compression chamber with its high oil pressure and the secondary low-pressure chamber there is integrated a „bypass“ with an adjusting valve. During impulse oil flows from the high-pressure chamber to the low-pressure chamber. By alterating the cross-section of this valve the performance (torque) of the wrench can be adjusted. The impulse sequence is about 10–40 impulses per second. The force transmission by means of oil reduces the noise level as well as vibration.

From Tightening Torque over Pretensioning Force to Clamping Force

The operational principle of a bolted joint exists in the compression of several construction units or components. Three quality-determining characteristics will interaction.

Torque means the physical force which affects over a defined lever right-angled on an axis of rotation (vertical rotation acceleration). The torque is measured in Newtonmeter (Nm = kg m²/s²) and is the cross product of lever arm of force times force.

Georgia State University says this about torque: "Note that the torque is maximum when the angle is 90 degrees. A practical way to calculate the magnitude of the torque is to first determine the lever arm and then multiply it times the applied force. The lever arm is the perpendicular distance from the axis of rotation to the line of action of the force."

Apply the same force to a different length lever arm and the torque output changes. The length of the lever arm includes the area on the head up to the center of the fastener.

By interaction with the screw thread the initial tension strength is produced. The pre-loading is the physical force, which is to hold the construction units together. It affects along the screw axis toward the construction units to be connected. Therefore the preload is the force that is supposed to hold the components together. From this the clamping force finally results, which compresses the components to be bolted on to each other and which during load are to behave as one component afterwords. The friction (static friction, self-locking) prevents the bolted connection from loosening automatically.

However, many interfering factors may affect the firmness of the bolt connection, e.g.

• Frictional loss
• Setting loss
• Component failure
• Constructional fault
• Manufacturing error
• Tool failure
• Faulty handling
• etc.

a step into future

The Toyota Production System (TPS) worldwide qualifies as benchmark for highly efficient assembly in most different branches of industry. Toyota successfully relies on impulse technology for years. Electronic controlled impulse wrenches of the Japanese specialist Yokota are not the tools for bolted joints of the future – they are the tools of today functioning like they should.

Whoever has watched the development of manual pneumatic nutrunners in the past decades, especially in the automobile and subcontractor industry, can realise a clear line from formerly used impact wrenches over shut-off tools right up to modern impulse wrenches. The advantages of conventional nutrunners – fast, lightweight and nearly reaction-free – had to be accepted with high noise level, vibrations, large torque tolerances plus the impossibility to measure tightening torque. The aggravation of statutory provisions, the manufacturer liability and the wish to use as small bolts as possible, which can be used up to yield strength, have made the application of impact wrenches more and more problematic. Further problems caused by the so-called re-torque shall be ignored at this point.

Later on increasingly used nutrunners with shut-off automatism accrued the advantages that torque was presettable in closer tolerances, that torque was measurable and documentable by transducers and electronic sensors, and that the noise level was considerably lower. Certainly it is useful for manufacturing when the tools stopp automatically at accurate torque and required clamp force, because the risk on human errors is reduced by automatic shut-off. The price to be payed for this advantage consisted of prolonged bolting times. Namely by lower speed of rotation, heavy weight and large dimensions, that generally necessitated two-hand operation due to the reaction of the tool on the operator at the instant of shut-off.

A solution for all these problems was promised then later on by the impulse wrenches increasingly coming onto the market. Beneath the high operation speed they also provide the control of torque. The tool itself is small, leightweight and applicable for one-hand operation. A further advantage is the reaction-freeness of the Yokota impulse wrench. Tissue disease like RSI will be avoided in comparison with other pneumatic nutrunners. Using Yokota impulse wrenches reduces afflictions of the operators and the corresponding financial risks to a minimum.

Primarily on mid-range torque values mainly applied in the automobile industry, the advantages of impulse wrenches are significant:

• Low noise level
• Low vibration
• High torque repeatability at high bolt speed
• Reaction-free

Because of the strong supporting moments with the angle nut runners these were replaced in the automobile industry nearly completely by other tightening systems. American and European manufacturers developed torque angle of rotation nutrunners with shut-off or with computer-control. The Japanese industry rated such tightening systems as unsuitable because of the high price, the large weight and the too high accuracy (!). The tightening time however is of far higher priority for the Japanese industry. Impulse nutrunners, with or without electronic control, in their understanding would by far fulfill the accuracy requirements. Moreover, such a system must be reasonably related to the quality of the screws and parts. It hardly makes sense to use highly exact screwing systems if the screws themselves have far larger tolerances and thus the completely outweighing portion of inaccuracies to cause. Regarding in the automobile industry occurring safety-relevant screw connections (A-joints), it is of crucially higher importance to record a 100% documentation of all safety-relevant screw connections in writing and/or dataelectronically for each mounted vehicle.

Computer-Controlled Impulse Tools

A prejudice, that is still widespread in the French and in the German automobile industry, exists in the opinion that the torque of an impulse nutrunner is not measurable with the torque measuring systems offered on the market, because they obviously were all voted on screwdrivers and angle nutrunners. Although it is outdated long, this before times correct argument also today is still stated by many users against the use of impulse nutrunners – particularly with safety-relevant screw connections. This opinion in practice however is verifiably disproved the measuring systems offered by Yokota as well as when using the recent test systems of the British measuring technique specialist Crane.

The problem was solved by the computer-controlled system nutrunner developed by the company Yokota – a system, which is based on torsion of the drive shaft, and conducting a group counting. For measuring the torque the Yokota system nutrunner uses a sensor in the pulse mechanism, which by means of strain gauge transforms the torsion of the drive shaft into electrical signals, which are then transmitted to the control unit. Torque and pulse number can be controlled, checked and printed out or exported to the computer with the YETC. The electronically controlled hydraulic impulse nut runner works with a very high number of impulses. Thus further advantages result:

• A screwing procedure transacted with a system nut runner requires approx. 2 seconds and can be implemented with one hand also still at a tightening torque of 600 Nm.
• Computer-controlled tightening systems offer short screwing times at low costs.

For the Japanese automobile industry it is characteristic that relatively simple however fast performing tools are used for assembly. The best proof for it is that the Japanese automobile industry in strong measure applies impulse nutrunners and screwing systems of Yokota.

Re-torque Angle of Rotation

In the automobile industry numerous torque wrenches are in use, which are used to "re-torque" bolt connections accurately on the demanded torque value. This processing step can be saved by a system nutrunner. Routine controlling of the bolt connections tightened with system nutrunners is randomly made with torque measuring wrenches. In the same way this is handled when using torque angle of rotation nutrunners. This approach to examine bolt connections from handheld screwing systems was generally usual and appreciative practice in the automobile industry. A meaningful relation between the torque accuracy and the firmness of a bolt connection however can only be made by determination of the apparent yielding point of a screw. This is technically possible, execution conditionally however high work expended and is therefore very cost-intensive. A further reason for the grasp to the Yokota impulse nutrunner is thus the maximum clamping force with minimum loosening risk.

With bolt connections clamping is most important – thus the axial tension in the pin, which ensures the cohesion of the parts. The way to the correct clamping force leads across the attitude of the starting torque after the characteristic values of the screw. In practice it shows up that after tightening rather high setting losses arise. Often this reduces in the long run the tension desired substantially. The experience shows, and lab tests confirm this, that the setting losses are substantially larger with a screw tightened with a angle nutrunner than after the use of an impulse nutrunner. Reason for it is that with the application of an impulse nutrunner while tightening the screw itself is shifted into a condition of vibration. Setting takes place here to a large extent already during bolting. In addition, with handheld tools the application of the angle of rotation is afflicted with many uncertainties.

Yokota differently solves the problem of pulling tight by a certain angle of rotation. Pulling tight takes place via some additional impulses after reaching the adjusted torque, the so-called afterpulses. Additionally between one to fifteen impulses can take place. Beside the vibration when tightening setting losses are compensated to a large extent when tightening by this afterpulsing. Hereby a substantial demand of the automobile industry is fulfilled. Moreover there is the possibility of hundred percent control and documentation. In addition optical and acoustic warning signals can be integrated separately into the system or together and it is possible to define so-called bolt groups. For example twelve screws can be defined as a group. If with tightening a bolt is forgotten then, the system refuses the transition to the next group of screws.

Recommendation

Only a 100 % documentation of all screw connections offers demanded security. All torque values must be traceable; only like that it is ensured that each screw was tightened. This means that impulse nutrunners should be used in the automobile industry in accordance with following recommendation:

• VDI 2862 category C: Standard impulse nutrunner with high torque repeatability
• VDI 2862 category B: Shut-off impulse nutrunner or Poka Yoke nutrunner
• VDI 2862 category A: Computer-controlled system impulse nutrunners

A meaningful and appropriate relation is ensured in this way between accuracy, screwing time and conditions of work for the operator. Yokota used up itself the continuous technical advancement, the observation of the production processes, the efficiency improvement as well as the increase of speed and security - for the advantage and benefit of the automobile industry.

according DIN EN ISO 6789

Denominated as „torque wrenches“ are manually operated tightening tools by which a defined tightening force (torque) can be applied to joint elements like bolts or nuts. Torque is a force exerted at a distance from the axis of rotation. Change either the distance from the axis of rotation or change the amount of force that is exerted and the torque changes.

According to DIN EN ISO 6789 torque tightening tools are differed into indicating (type I) or releasing (type II) models.

Type I indicates the torque value by a mechanical scale, a dial gauge or an electronic display (“measuring wrench”):

• Class A: with torsion bar or beam lever
• Class B: with scale, dial or digital display
• Class C: with electronic measuring
• Class D: screwdriver with scale, dial or digital display
• Class E: screwdriver with electronic measuring

Type II is to be preset to a certain set point activating a signal as soon as the set value is achieved (“signaling wrench”):

• Class A: adjustable, with scale/digital display
• Class B: preset
• Class C: adjustable, without scale
• Class D: screwdriver, adjustable, with scale or digital display
• Class E: screwdriver, preset
• Class F: screwdriver, adjustable, without scale
• Class G: with beam lever, adjustable, with scale

Specified Measuring Range

The requirements and verification procederes of the DIN EN ISO 6789 are valid for a measuring range from 20% to 100% of the rated capacity (maximum value). The effective span of a torque tool may differently begin lower.

Allowed tolerance type I

• Class A, D: ± 6%
• Class B, C E: upto 10 Nm ± 6%, beyond ± 4%

Allowed tolerance type II

• Class A, B, C: upto 10 Nm ± 6%, beyond ± 4%
• Class D, E, F, G: ± 6%

Required marking

According to DIN EN ISO 6789 the manual torque tools delivered by us are durable and easily readable marked with at least following information:

• Rated capacity (maximum torque)
• Unit
• Direction (if not bi-directional applicable)
• Manufacturer or brand
• Serial number (as far as calibration cert. is included)

of torque application tools according to DIN EN ISO 6789

For the calibration of manually operated torque tools the international standard EN ISO 6789 is decisive (2003). Accordingly calibration has to be understood as those “activities which under given conditions determine the mutual allocation between the stated or indicated values of a torque application tool and the associated values of a calibration device.” Aim of the measures is the appraisal and documentation of the difference amount between given value and actual value of a torque tool using calibrated measuring instruments.

Re-calibration

According to EN ISO 6789 defined as “the requirements to be fulfilled during the calibration of manually operated torque application tools to a defined service life.” Hence torque wrenches are to re-calibrated in a determined interval periodicly.

Procedure

Torque application tools must be examined according to calibration conditions described in EN ISO 6789 point 6.3 and fulfill the requirements of point 5.1.5 (tolerance limits). For closer information on this please consult EN ISO 6789.

Calibration device

The maximum permissable measuring inaccuracy of the calibration device amounts to 1% of the indicated value. The measuring inaccuracy is to calculated according to the “Guide for to calculate evaluation OF Uncertainty in Measurement” (GUM) with an extension factor k = 2. For closer information on this please consult EN ISO 6789.

Interval

Torque application tools are to be regarded also as inspection devices. If the user accomplishes procedures for the control of inspection, measuring and test equipment, the torque application tools must be included into these procedures. The calibration interval is to defined due to certain usage factors, e.g.

• required accuracy
• frequency of usage
• typical load during usage
• environmental conditions during procedure
• storage conditions

The period is to be adapted according to the defined procedures for the control of inspection, measuring and test equipment and under evaluation of experiences won by the re-calibration. If the user does not accomplish procedures for the control of inspection, measuring and test equipment, a life of 12 months or approximately 5,000 loads can be taken as reference value for a re-calibration interval. For the first re-calibration the period of the validity begins with the first use of the torque application tool.

Calibration Service

Maintenance, calibration and adjustment are accomplished in our internal workshop according to EN ISO 6789. We offer also servicing contracts for those date-supervised calibration of your torque wrenches. At interest contact us please. We will gladly inform you in detail.

The lever length affects the tightening torque

Torque is a force exerted at a distance from the axis of rotation. Change either the distance from the axis of rotation or change the amount of force that is exerted and the torque changes.

Add an extension on the end of a torque wrench and you have changed the distance from the rotational axis. That same effect is created when changing the length of the head you are using. That too changes the distance from the rotational axis.

The common centerline is measured from the center of the fastener to the top of the dovetail. Thus with the application of torque the common center line is a part of total lever length and for this reason an important characteristic.

The image to the right illustrates the concept posed above. Here are two preset click wrenches that are set to the same value. When they click they will provide a different torque value at the fastener because their common centerline is not identical.

Therefore tool heads with same common center line can be changed arbitrarily among themselves, without needing the click wrench to be adjusted again. Therefore it is compellingly necessary with different common center lines to calculate the torque again in each case and/or to adjust the click wrench again in each case.

If your application requires a preset wrench with a 60 mm open end head, it can be difficult to attach the head of the wrench to your tester. SR dovetail 3/4 ratchet and 3/4 solid square drive have the same common centerline as the 60 mm open end head. By switching to the 3/4 drive, calibration is now possible.

Due to physical size constraints, Sturtevant Richmont interchangeable heads have two common centerlines. Typically for sizes 32 mm and below the common centerline is 36.5 mm (1 7/16 inch). For sizes larger than 33 mm the common centerline extends to 98.4 mm (3 7/8 inch).

Calculation example when using an extension:

for standard metric threads

The torque values indicated here (in N·m) are valid appoximate values for metric standard threads (ISO threads) according to DIN 13 and head seat admeasurement according to DIN 912, 931, 934, 6912, 7984, 7990. They result in an approx. 90 percent utilization of the screw apparent yielding point.

Pay attention that the coefficient of friction depending upon surface quality of the screws and nuts as well as the lubrication condition of the thread have possibly substantial influence on the aforementioned values and must be corrected.

Thread statements of size for pneumatic nutrunner rated outputs refer generally on screws of the grade 8.8 at 6 bar flow pressure and a pulse duration of 2 seconds. With the use of screws of smaller grades caution is required concerning tearing the screws off. On strongly soft joints the indicated bolt achievement is seized perhaps too highly and stronger screws are necessary.

caused by repetitive monotone motion-sequences

Disease Pattern

• The Repetitive Strain Injury syndrome has a complex clinical picture. It evolves from a chronic degradation of the musculoskeletal system particularly in region of hand, arm, shoulder and neck due to long-lasting monotone work movements. Thus it causes repeated micro injuries of muscles, tendons, ligaments, articulations and/or nerves. These could heal completely first still without consequences, during continued stress in the course of the time however usually resulting via scar formation to chronic disorders.
• RSI initially manifests in loss of energy, joint rigidity and paraesthesia as numbness or tingle, however it might lead to co-ordination disturbances of the arms and hands. Pain arises usually in the advanced stage.

Concerned Persons

• RSI is diagnosed mainly with humans, who always (must) make the same movements again and again over longer time, e.g. workers at the assembly-line, cashier, secretary or computer scientist and all other office employees at computer monitors (“mouse arm”).

Occupational Disease

• In the United States RSI is accredited as appreciative occupational illness for different occupations since end of 1998. As appearing from a report of U.S. National Academy of Sciences to the American parliament, the causal relationship between stereotyped courses of motion on the job and musculoskeletal health trouble has to be understand as scientifically proven fact.

Prophylaxis

• In the modern working sphere constantly returning courses of motion are frequent. As a condition for a successful RSI prevention a good work attitude and good ergonomic conditions of work are of greatest importance. Certain valid guidelines were issued from the legislator and by different organizations.
• At the assembly line the RSI risk lets itself reduce significantly by the use of reaction-free Yokota impulse tools.

Further information about RSI

• Computer Related Repetitive Strain Injury
• Repetitive-Strain-Injury
• RSI Action
• Wikipedia

for different torque units

of former wrench models