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In Focus - Archives Août 2008
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01.08.2008 -
We are frequently confronted with torques on a daily basis, even though we do not recognize them as such.
For example, there is torque in a door lock when the door handle is pushed down. There is torque in the cap of a bottle containing a carbonated beverage when the cap is unscrewed. When a chair is tipped back, torque is also at play, since a force is acting on a pivot point. Torque occurs whenever one or more forces are exerted and there is a common pivot point.
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What is clamping force / torque?
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When a force acts on a rigid body, it is translated into motion, or its speed changes.
If the body is held at one point, then no translation motion is possible. The ability of the body to move is then reduced to rotational motion (turning) on this point. The variable that affects this rotary motion, i.e. which causes the change in the rotational speed, is called torque.
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A single force F1 cannot cause a purely rotational motion. To affect the torque without changing the translation motion, an additional force F0 (retaining force) is required in addition to the active force. This second force is applied here by rotational fastening of the body.
In order for the fastening means to prevent the translation motion, i.e. to allow only rotational motions, the opposing force applied by the fastening means has to be exactly equivalent to the acting force: F0 = F1.
In addition to the variable of the two forces F1 and F0, the distance between the two points at which the forces act also affects the rotational motion. The distance r is a vector, which is directed from the point of action of force F0 to the point of action of F1. The torque is affected only by the component r’ of r, which is perpendicular to the direction of the force F1 (or F0).
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Torque = lever arm * force
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r' is the distance at which both forces act. The amount of torque is then the product of F1 and r', and the direction of the torque is perpendicular to the plane that is held by the force F1 and the distance vector r, namely in the direction in which the thumb points when the bent fingers of the right hand (right-hand rule) point in the direction of the rotational motion caused by the torque. This relationship between the forces acting on the body, the distance vector of the two points of action and the torque (both amount and direction) is expressed in short form by the cross product or vector product.
This figure produces the following definition for the torque M:
M = r * F
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Right-hand rule
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Or in simple terms:
The force applied to a lever arm causes a turning motion. The longer the lever and/or the greater the force, the greater is the turning force or torque.
Quote: "If you give me a long enough lever, I can lift the earth from its foundation."
The physical dimension of the torque is therefore the product of the force and the distance. In the SI system, it is the (derived) dimensional unit known as the Newton meter (Nm = kg x m2 / s2).
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What is the implication of this? What is the effect of the torque?
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Example 1:
Torque based on the principle of a hand of a tower clock:
Assuming the one meter long hand of a tower clock weighs so much that there would be a force of 10 kilograms at the end, there would be a torque of 100 Nm at the center of the clock.
Calculation (simplified):
F = 10 kg = 100 N
l = 1 Meter
M = F * l = 100 N * 1 m = 100 Nm
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The clock bearing then has to be designed so that the hand is held with a force corresponding to a torque of 100 Nm. A higher holding force causes a drive effect in the torque of the hand, for example when a bird alights on the hand.
This would result in the following for an extended calculation.
Calculation with bird (simplified):
F = (10 kg + 250 g) = 102.5 N
l = 1 Meter
M = F * l = 102,5 N * 1 m = 102.5 Nm
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Example 2:
A further practical example to illustrate the torque is the loosening of a tightened bolt.
If a wrench with a length of one meter is used to loosen a bolt, a torque of 100 Nm (100 n * 1m) can be applied by pressing the end of the wrench with a force of 100 N. The bolt has to apply a retaining force of 100 N in the opposite direction, which can cause the bolt to tilt or bend.
This situation is intensified with a shorter wrench. To apply the same torque with a wrench that is half as long, an acting force and an opposing force of 200 N is needed (200 N * 0.5 m). This additional strain on the bolt can be completely eliminated by using a wrench with a hexagon head that is located at the center of the lever arm of the wrench. If a force of 100 N is applied to both ends of this wrench in opposing directions and the lever has a length of one meter, then a torque of 100 Nm is applied here (100 N * 0.5 m + 100 N * 0.5 m), however without the bolt having to apply any retaining force. If such a wrench is not available, then the strain on the bolt can also be relieved by duplicating the same force applied at the far end of the lever to the other end (near the bolt) in the opposite direction.
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Other torque variants:
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The force F0 that opposes the force F1 does not have to be applied externally. (Example: cutting a ball by kicking it on the side.) F0 can also be applied by the inertia of the body. This inertia force occurs only in case of acceleration of the body, resulting in superposition of the translational and rotational motions.
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Salvaging an automobile
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Is the torque relevant for toolholding?
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The torque in a toolholder is defined at two positions. The toolholder taper is a double mechanical interface between the cutting edge and the main spindle, separated into two completely different holding setups.
On the one hand, the toolholder provides a rigid, positioned connection with the driven machine spindle, and on the other hand with the metal-cutting tool. An important specification point at this interface between the toolholder and the tool is a sufficient clamping force. The torque response at the interface between the spindle and the toolholder is of secondary importance, since form-fit clamping is generally given --> grooves on the toolholder and T-nuts on the spindle.
For the interface between the toolholder and the tool, milling processes are characterized by severely alternating machining forces, which usually are applied at the outermost lower flank of the cutting edge. To control the torque, universal toolholders with high indexing accuracy are used, which enable firm holding not only of milling cutters, but also of drills and reamers, for example.
The diversity of available products with their typical pros and cons makes the choice difficult. Some of the decisive factors include:
- vibration damping
- balance quality
- concentricity
- radial stability
- workpiece accessibility,
to name only a few.
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The transmission of torques in the toolholder is achieved with different toolholder versions:
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There are screw-in tools that are screwed into the base body via a threaded insert at a defined torque. This enables a secure connection by calculating the torque that will result from machining. The design of the threads has to match or exceed this calculated torque.
Mechanically clamped tools have the advantage that they are usually equipped with a "non-positive drive". This means that the clamping will fail only when the tensile strength or shear strength of the component is exceeded. These strengths are generally far above the torques that occur during machining.
Positive toolholding systems make use of lateral pressure connections. The connection (usually the bore that holds the tool) is established by means of heat or force so that the tool is held as a result of overlapping.
Of course, there are limits to the degree of expansion that is possible, defined by the strength and thermal properties of the material used for the toolholder. The torques here are also significantly lower than those in screw-in or clamped toolholder connections. The advantages of positive connections are usually to be found elsewhere, such as excellent true running properties and a smooth shank that is easy to manufacture. The primary requirement here is not the torque.
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Total Tooling
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These toolholders from the SCHUNK TOTAL TOOLING product line feature a very high torque and maximum clamping forces, which makes them ideal for rough machining.
Large selection of precision toolholding systems
The TOTAL TOOLING line from SCHUNK offers several precision toolholding systems that are suitable for rough machining, in particular TRIBOS-R, SINO-R and the toolholders of the MEGA series from BIG.
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SINO-R universal toolholder
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The SINO-R universal toolholder is especially suitable for rough machining. SCHUNK designed SINO-R as a universal toolholder based on expansion technology. The design is innovative and absolutely price-optimized. In terms of quality, flexibility and cost savings, these universal toolholders offer convincing features as compared with conventional toolholders such as Weldon, Whistle Notch or ER collet style toolholders. SINO-R, with its solid-state medium, achieves maximum clamping forces and even, full-surface clamping action. Tools are held with torques of up to 850 Nm (tool shank clamping diameter 32.0 mm, shank quality h6).
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TRIBOS polygonal clamping technology
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TRIBOS polygonal clamping technology is a further patented in-house development from SCHUNK that covers a broad spectrum of different size and system variants. TRIBOS can handle everything from micro-machining with shaft diameters starting at 0.3 millimeters to rough machining. The areas of utilization range from the automotive industry to precision engineering and medical technology. TRIBOS-R scores especially high in this sector with maximum clamping forces. TRIBOS-R ensures reliable and powerful clamping. High infeed rates and therefore high machining volumes are possible. This not only saves time, but also reduces costs significantly.
Specially designed for micromachining is the TRIBOS-RM toolholder from the TRIBOS family. Also featuring maximum clamping forces, TRIBOS-RM can withstand the loads of HSC machining centers. High cutting performance speeds up machining times for increased productivity.
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Big Mega New Baby Chuck
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Precision collets from BIG in Japan round out the line of toolholders with maximum clamping forces and torques from SCHUNK. The needle bearing design of the clamping nut enables the transmission of maximum turning torques with low tightening torques.
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12.2008
Quick jaw change brings flexibility and substantial savings
Flexibility in production is a crucial competitive advantage. Only companies that can respond to demand with flexible yet absolutely precise production can supply their customers rapidly with a constantly increasing variety of products without needing to build up expensive and inefficient stocks.
 Pour en savoir plus…
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11.2008
Pneumatics or mechatronics? … It all depends!
To date, pneumatically driven gripper modules have been used primarily in gripper technology. Recently, however, more and more electric grippers have been coming onto the market. Both competing systems have powerful arguments in their favor.
Pour en savoir plus…
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10.2008
High-Performance-Cutting (HPC)
Hard on the outside, clever on the inside: Toolholder systems developed especially for HPC that have strong rigidity as well as good vibration dampening provide clear benefits for HPC.
Pour en savoir plus…
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09.2008
Cleanroom
The importance and the use of cleanrooms in assembly and handling are constantly increasing.
Pour en savoir plus…
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08.2008
Clamping Force / Torque
We are frequently confronted with torques on a daily basis, even though we do not recognize them as such.
Pour en savoir plus…
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07.2008
Convenient robot or attractively priced spindle gripper?
A significant potential for increasing efficiency in modern machining processes is the automated loading and unloading of the machines.
Pour en savoir plus…
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06.2008
Polygon clamping technology - round pegs in polygonal holes
The principle behind polygon clamping is surprisingly simple. The know-how that makes it work is however highly advanced.
Pour en savoir plus…
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05.2008
Packaging
A package for everything
Packaging is a universal function that is required in practically every industry and meanwhile is so important that it has become a separate industry itself.
Pour en savoir plus…
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04.2008
Magnetic clamping technology has strong attraction
Workpiece clamping with electroermanent magnets reduces set-up times by up to 50 percent and brings about significant advantages especially in the manufacture of large parts.
Pour en savoir plus…
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03.2008
Quick-change systems speed up production
Changing systems for components such as grippers and tools add a high degree of flexibility to production processes
Pour en savoir plus…
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02.2008
Vibration damping
Improves surface quality and reduces wear on the tool and spindle.
Pour en savoir plus…
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In Focus - 2013
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