- What is torque?
Torque is the measurement of the tendency of a force to rotate the body upon which it acts about an axis.
- What is bolt load or bolt tension?
The act of stretching tight or state of being stretched tight. Stress of a pulling force on a body.
- What is preload?
Initial clamping force or tension in a fastener.
- What is stress?
Stress in force exerted against each other by two surfaces in contact.
- What is the short formula?
T = K x S x A x D
- What is the “K Factor”?
The K Factor is an experimentally determined constant that relates the torque applied to the load induced in the fastener. This factor is affected by the condition of the fastener, the lubricant used and the condition of the flange. The K Factor s based on the following conditions:
- Condition of flanges, studs and nuts.
- Thorough application of lubricant on all mating surfaces of flange, nut and stud.
- Type of washers used.
- How much torque should I use to tighten a specific bolt?
To answer this, a few facts are needed about the application in question:
- What is the bolt diameter?
- What is the bolt material?
- What lubricant is being used?
- How much bolt load is required?
You can estimate the torque requirement by using these facts in the “Short Formula”: T = K x L x D
- Is tighter always better?
No. Bolts are designed into systems for specific reasons. Too much torque and/or bolt load can be as bad as too little.
- Why should I lubricate bolts and nuts?
First, lubrication helps prevent galling or seizing when tightening and especially when loosening after being in service. Secondly, less torque is required to tighten (create bolt load) when a good lubricant is applied. Less work equals same results! And lastly, lubricants help prevent rust and corrosion.
- Why do bolts loosen in service?
There are several factors that can lead to this:
- Gasketed joints may loosen if the gasket compresses after bolt preload is applied.
- Vibration may create dynamic loads greater than preload. This can allow the bolt to turn and loosen.
- Temperature changes during service can cause bolts to “grow” or “shrink” relative to surrounding parts. Bolt load may lessen and allow loosening or leakage.
- Why is it required to use a 'star' or 'criss-cross' pattern and two or three passes when tightening bolts in a pattern?
These methods are used to apply uniform bolt load in each bolt. The designer has specified the size and number of bolts to secure the parts of an assembly. If some bolts are tightened to a greater bolt load than others; they may carry a greater load in operation, causing a bolt failure.
Product Specific Q&A
Direct Tension Indicator (DTI)
- What is a Direct Tension Indicator?
The Director Tension Indicator (DTI) is a specially hardened washer with protrusions on one face. The DTI is placed under the bolt head or nut, and the protrusions create a gap. As the gap is tensioned, the clamping force flattens the protrusions, reducing the gap. For complete information, review our spec sheet and user’s manual.
- Is it possible for my company to get a demonstration of the Direct Tension Indicators?
Yes! Fastorq specialists are happy to provide additional information and demonstrations at our location or yours. For an immediate review of how DTIs work, check out this video by Fastorq. (Link to: http://youtu.be/s677UKFiRdA)
- Has there been any testing to confirm the information on DTIs?
Yes. Visit our Technical Reports section for DTIs ( ). If you have additional questions, feel free to contact a Fastorq specialist at 800-231-1075 (US only) or 281-449-6466.
- Will using DTIs change my torque or tension requirements?
No. DTIs measure bolt load, but do not change requirements.
- If there are great temperature fluctuations in the joint, will DTIs cause relaxation in the bolt load?
No. If the fasteners are tensioned according to specification, temperature creep may still occur but the DTIs neither increase or decrease it.
- If the stud/bolt is over-tensioned, can I back the nut off and use the same DTI again?
No. Once the protrusions on the DTIs are compressed past the designated amount, a new DTI must be used.
- If enough clamping force has not been achieved when the gap is measured, do I need to start from the beginning again?
No. If the “no go” gauge still fits in the gap, simply create more tension (and therefore more clamping force) until the “no go” gauge does not fit but the “go” gauge still does fit.
- Why should I use a lubricant?
Friction between mating threads and between nut face and flange absorbs about 90% of the energy used to torque (tighten) a threaded fastener (bolt), 10% of the energy creates bolt pre-load. Reducing friction by using a better lubricant reduces the amount of energy (work) required by a factor of ten to one.
- Does lubricant cause bolted joints to be come loose?
No. If the bolt is tightened to the required preload (greater than the dynamic load experienced in service), the lubricated bolt will not vibrate loose.
- How can I prevent bolts and nuts from freezing up (galling) when tightening or taking them apart?
Threads gall due to metal to metal contact between thread surfaces. To avoid galling use a lubricant with a high percentage of solids which will remain on the threads during the service life of the system. Choose a lubricant with a temperature rating higher than the temperature experienced by the bolt in service.
- If the lubricant is too slippery- won't the nut loosen more easily?
No, a low coefficient of friction of the lubricant by itself will not cause loosening unless dynamic forces are present which momentarily reduce the preload and subsequently the friction in the bolt and allow the nut and bolt to turn relative to each other. Dynamic forces may be created by vibration or temperature change among others. If preload is greater than the loads created by the dynamic forces, bolt load loss (loosening) will be avoided or at least minimized.