Unit of measurement kgf (kilogram-force) is still found in technical documentation where it is necessary to indicate the tightening force of suspension bolts or the pulling force of a winch, and confusion with it most often arises when trying to compare the old instructions with a modern torque wrench marked in Newtons. Unlike a kilogram of mass, which is a constant value, kgf depends on the acceleration of gravity, which makes this value critical for accurate engineering calculations of the strength of components and mechanisms. Understanding the physical nature of this quantity allows you to avoid fatal errors when assembling an engine or setting up a brake system, where under-pressure or pinching of a threaded connection can lead to destruction of the part.
When working with Soviet-made cars or specialized industrial equipment, you will regularly encounter designations that require translation into the international system SI. Ignoring the difference between the mass of the load and the force with which it presses on the support often leads to the wrong choice of tool or fastener. For example, if the specification indicates a force of 50 kgf, and you tighten a bolt based on a weight of 50 kg, without taking into account the gravitational component, the actual force may differ from the required one, which will affect the reliability of the unit.
The main challenge for the modern mechanic is that most digital torque wrenches and electronic catalogs use the unit Nm (Newton meter), whereas in old GOSTs and manuals it dominates kgf m or just kgf for linear forces. Incorrect conversion of these values when adjusting timing belt tension or adjusting valve clearances can lead to accelerated wear of parts or their sudden breakage under load. Therefore, the skill of instant translation and understanding of the physical meaning of these quantities is mandatory for a qualified specialist.
Physical essence and definition of kilogram-force
The kilogram-force is a unit of force in the IKGSS technical system of units, which was widely used in engineering before the system was widely adopted SI. From a physical point of view, 1 kgf is the force with which a body weighing exactly 1 kilogram acts on a support under the conditions of the standard gravitational field of the Earth. This means that the value is directly related to the acceleration of gravity, which is generally considered to be approximately 9.80665 m/s².
It is important to understand the fundamental difference between mass and by force, since in everyday life these concepts are often confused, calling weight in kilograms. Mass is a measure of the inertia of a body and does not change depending on location, while force, measured in kgf, can theoretically vary with changes in the gravitational field, although this is neglected for terrestrial conditions. In automotive engineering, this difference is critical when calculating loads on the frame, suspension and body elements.
The relationship between the kilogram-force and the basic SI unit of force, the Newton, is expressed through an exact conversion factor. One Newton is defined as the force that changes the speed of a body weighing 1 kg by 1 m/s in one second. Since the gravitational acceleration $g$ is approximately 9.81 m/s², the ratio between the units is fixed and does not depend on the type of car or engine.
⚠️ Attention: Never use the value of 10 m/s² for precise engineering calculations of critical components such as the cylinder head or connecting rod bolts, as an error of 2% may be outside the manufacturer's tolerance.
Mathematical conversion to SI system and vice versa
To work correctly with technical documentation, you must firmly know the conversion factor that binds an outdated but tenacious unit kgf with modern Newton. The conversion formula is as follows: the force in Newtons is equal to the value in kilogram-force multiplied by the acceleration due to gravity. For most practical tasks in a car service center, a rounded value of 9.81 is used, which provides sufficient accuracy.
If you need to transfer the moment of force, for example, when tightening bolts, then the formula is complicated by adding a lever arm. Moment of force in Newton meters obtained by multiplying the value by kgf m at 9.81. When working with small values such as kgf cm, which are often found in carburetor adjustments or small electrical connections, decimal point offset must be taken into account when converting to Nm.
Back translation is also important, especially when working with imported instruments calibrated in pound-feet or Newtons, and domestic documentation in kgf. In this case, the value in Newtons is divided by 9.81. To simplify mental calculations, many mechanics use an approximate factor of 10, but this is only acceptable for non-safety estimates.
Exact conversion factors
1 kgf = 9.80665 N|1 kgf m = 9.80665 Nm|1 kgf/cm² = 0.980665 bar
Below is a table that will help you quickly navigate the basic values often encountered when servicing cars and trucks. Keeping this data on hand will simplify your work and eliminate the need to turn to the calculator every time.
| Value in kgf | Value in Newtons (N) | Typical automotive application |
|---|---|---|
| 1 kgf | 9.81 N | Basic unit of force |
| 10 kgf | 98.1 N | Drive belt tension |
| 50 kgf | 490.5 N | Spark plug torque |
| 100 kgf | 981.0 N | Fastening suspension elements |
Differences between mass, weight and strength
Confusion between the concepts of mass and weight is the source of many errors when reading specifications. Weight measured in kilograms (kg) and characterizes the amount of a substance in the body; it is unchanged on Earth, on the Moon or in zero gravity. At the same time weight - this is the force with which the body acts on the support, and it is this that is measured in kgf or Newtons.
Under the conditions of the earth's surface, we often say “weight 100 kg”, meaning mass, but from the point of view of physics and technology this is incorrect. If you were taking measurements on another planet, the mass of the car would remain the same, but its weight would be kgf would change in proportion to the local acceleration of gravity. For earthly workshops this difference is not significant, but the principle is important to remember.
When the instructions indicate “a clamping force of 5 kgf,” this means that a force equivalent to the weight of a load weighing 5 kg must act on the part. If the “mass of the part is 5 kg” is indicated, then to calculate the load on the suspension or foundation, this mass must be multiplied by the gravity coefficient to obtain the required force.
Key difference: Mass (kg) is a property of a body, and Force (kgf, N) is a measure of the influence of one body on another.
Application of kgf in automotive technology
In the automotive industry, one kgf historically used to refer to a variety of parameters, from tire pressure to engine torque. Although manufacturers are gradually moving to standards SI, in garages you can still find pressure gauges with a double scale or instructions where the pressure is indicated in kgf/cm² (technical atmosphere).
One of the most important applications is monitoring the tightening force of threaded connections. Cylinder head bolts, connecting rods, bearing caps and wheel nuts have strictly regulated torque values, often specified in kgf m. Exceeding this value leads to thread stretching or bolt breakage, and under-tightening leads to spontaneous loosening due to vibration.
Also in kgf often measure the tension force of the generator and timing belts, the force on the brake and clutch pedals, as well as the traction force of winches. Understanding that 100 kgf is a tangible force that not every person can develop with their hands without a lever helps to choose the right tool for the job.
- 🔧 Torque wrenches often have switchable scales, allowing you to work directly with Nm and kgf m.
- 🚗 Tire pressure in old gauges is indicated in kgf/cm², which is numerically close to atmospheres (at).
- ⚙️ The engine torque in the technical data sheets of old cars could be indicated in kgf m along with Nm.
Measurement tools and calibration
To measure forces in kgf or their analogues in the system SI specialized instruments are used that require regular checking. The main device is a torque wrench, which can be mechanical (arrow) or electronic. Pointer models often have a double scale, which simplifies the life of a mechanic working with heterogeneous documentation.
Electronic keys allow you to switch units of measurement with the press of a button, automatically converting kgf m in Nm and back. This reduces the risk of human error when converting, but it is important to ensure that the instrument is calibrated in the exact system of units you plan to use for final inspection.
When working with spring dynamometers used to measure linear force (for example, cable tension), the scale can also be graduated in kgf.
☑️ Check before tightening
⚠️ Attention: Using extensions on a torque wrench changes the lever arm and requires recalculation of the tightening torque, otherwise the actual force will be kgf will differ from the specified one.
Common mistakes and expert recommendations
One of the most common mistakes is to equate 1 kgf to 10 N to simplify calculations in critical units. Although the difference seems small, on the scale of the cylinder head tightening force, where the count is tens and hundreds of Nm, the accumulated error can lead to uneven pressure on the gasket and gas breakthrough.
Another problem is confusion between kgf m (kilogram-force-meter) and kgf cm (kilogram-force-centimeter). The difference between them is tenfold: 1 kgf m = 100 kgf cm. An error in the same order when adjusting valves or tightening small screws can lead to broken fasteners or damage to the body of the part.
Experts recommend always double-checking the units of measurement in the instructions before starting work, especially if the documentation is translated or scanned, where the symbols may not be clear. It is better to spend a minute on clarification than to redo the work or eliminate the consequences of the destruction of the unit.
Tip: For a quick mental calculation, you can consider 1 kgf m equal to 10 Nm, but always perform the final tightening using the exact value (multiplied by 9.81).
In conclusion, it is worth noting that although the world is gradually abandoning the system MKGSS in favor SI, knowledge of kgf what is this unit, remains an essential skill for any professional. The old vehicle fleet, specific equipment and the inertia of technical documentation provide this unit of measurement with a long life.
What is the difference between kgf and kg?
Kg (kilogram) is a unit of mass that does not change. Kgs (kilogram-force) is a unit of force that depends on gravity. On Earth, 1 kg of mass creates a force of approximately 1 kgf.
How to convert 50 kgf to Newtons?
You need to multiply 50 by 9.81. The result will be 490.5 N. For rough calculations, you can multiply by 10, you get 500 N.
Why do old books write kgf/cm², but now bar?
Kgs/cm² is a technical atmosphere, an outdated unit of pressure. 1 bar is approximately equal to 1.02 kgf/cm², so the values are very close and are often equated in everyday life.
Is it possible to tighten bolts “by eye”, knowing how much it is in kgf?
Strictly not recommended for critical connections (cylinder head, wheels, suspension). The human sense of effort is too subjective and inaccurate.