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Mechanical Clamping Technology
  
Mechanical clamping versus hydraulic clamping

Many factors should be taken into account when deciding whether to use mechanical or hydraulic workholding products for clamping your parts. In general, hydraulic clamping should be used in high volume applications, or when critical tolerances need to be held. Mechanical clamping products can be used in shorter production runs, or on rougher procedures where surface finishes and tight tolerances are optional.

For example, using hydraulic workholding products will allow you to maintain within a 1% accuracy on your clamping force. This is through the use of digital pressure switches, electric powered pumps and hydraulic clamping and support cylinders. This type of accuracy may be necessary when machining a surface requiring tight tolerances, less than .001 inch (0,025 mm). The slightest variation in clamping force could result in part movement or deflection greater than the required overall tolerance (Figure 1). In situations like this, the investment in hydraulic clamping is undeniable.

Mechanical Clamping Technology - Figure 1 Mechanical Clamping Technology - Figure 2

Mechanical clamping products are sufficient when tight tolerances are not required, or when the part is a large casting for example, and no amount of clamping force will distort the part. A typical operator, for example, can tighten a stud over a clamp to a specific torque value with possibly only 10% accuracy using a manual wrench. This could result in significant differences in part height and position on a fixture (Figure 2). However with a rough casting where the required finish is not critical, this may be acceptable. And, for the cost of mechanical clamping compared to hydraulic clamping, the choice is easy.

There are also situations where hydraulic clamping is not only not necessary for accuracy, but also, potentially dangerous. A perfect example of this is a die casting machine. Heat is an enemy of hydraulic components, and die casting obviously generates an enormous amount of heat. Mechanical clamping is an excellent and safe solution to the problem.

Production quantity runs should also be taken into account along with time savings and cost of materials when choosing between hydraulic and mechanical clamping.

Mechanical clamping is typically less expensive but more time consuming compared to hydraulic clamping.

See the examples below for ideal situations in which to use hydraulic or mechanical clamping:

Example 1
	Production quantity:	60,000 pieces
	Part material cost:	$25
	Machine time cost:	$150 p/h
	Hydraulic fixture and
	component cost:		$30,000
	Parts per fixture:	4
	Load/unload time:	20 seconds
	Run time:		720 seconds
	
The run time and the load/unload time equate to 185 seconds of machine time per part. The machine costs money no matter whether you are actually cutting chips or waiting to cut chips while you are loading the parts. This is why you must take both the load and the run time into account.

This 185 seconds per part equates to being able to run 155 parts per 8 hour day, at an additional cost of $7.71 per part due to machine time cost of $150.00 per hour.

The hydraulic fixture cost of $30,000 divided over 60,000 parts equates to an additional $0.50 per part. All together, in this very simple example, you have added only $8.21 to the cost of the part. The $8.21 equates to only about a 33% increase in cost. Granted, there are more aspects which could be factored in, but you can see the minimal cost added by hydraulics in this example.

Assume that you were only running 3000 parts on a small run. The machine time is the same, but now, the hydraulic fixture and components adds an additional $10 to the cost of the part (30,000/3000 parts). This is a total of $17.71 additional cost, or a 71% increase. Hydraulic clamping is much too expensive for such a short run.

Example 2
	Production quantity:	3000 pieces
	Part material cost:	$25
	Machine time cost:	$150 p/h
	Mechanical fixture and
	component cost:		$5000
	Parts per fixture:	4
	Load/unload time:	240 seconds
	Run time:		720 seconds
	
In this example, the production quantity is much lower, and mechanical clamping is being used. The same part is being machined, on the same machine process. The mechanical clamping fixture is much less expensive, only adding $1.67 to the cost of each part. However, the load/unload time has increased significantly since the operator has to manually clamp each part. The machine is now only able to produce 120 arts per 8 hour day. This adds $10 to the cost of each part in machine time cost. All together, $11.67 has been added to the cost of each part, a 47% increase. While this may seem significant, remember that the cost increase using hydraulic clamping was 71%. Mechanical clamping is a much better choice in the lower production runs, even though it may be slower.

Many factors must be taken into account to decide on either mechanical clamping or hydraulic clamping. For example, taking labor into account can significantly add to the cost of mechanical clamping, since it is a much slower process. These examples are very simple and do not include all of the variable details that could affect your decision. Be sure to account for every situation in making your choice.

Replacing mechanical clamping with hydraulic clamping
In order to properly replace a mechanical clamping set-up with hydraulic cylinders, the most important thing to understand is the amount of clamping force being applied to the part. Figure 3 is an example of a typical mechanical clamping set-up for either one part or two parts. In this situation, the operator tightens the nut on the clamping stud, which in turn applies a holding force to the work piece. In order to convert this set-up to hydraulic clamping, you will need to know some values from Figure 3.

Mechanical Clamping Technology - Figure 3

	T =	Torque on the clamping stud
		(ft-lbs or N-m)
	
	D =	Thread diameter and pitch
		(for example, 3/8-16 or M8)
	
	L1 =	Distance from center of clamping
		stud to contact point on the
		workpiece
	
	L2 =	Distance from center of clamping
		stud to reaction point (or contact
		point on second workpiece)
	
You will also need to know whether the clamping stud and nut are lubricated or dry. This makes a difference in how much clamping force is generated.

The first thing to know is how tight that nut is being applied to the clamping stud. This is best measured using a torque wrench. Even though the operator may not use a torque wrench in the everyday use of the fixture, it is critical to be able to provide a torque reading when converting to hydraulic clamping.

It may be necessary to use a torque wrench on the part a few times in order to get a good consistent value to be used in calculating the clamping force.

Once you have determined the amount of torque being applied to the clamping stud, and you have measured the diameter of the stud, and the distances L1 and L2, the clamping forces can be calculated. It is important to understand that the amount of clamping force being put into the clamping stud is not the same amount of force being applied to the part. In this setup, much less force gets applied to the part. You can calculate the force applied to the stud using the table. The force applied to the part is based on the formula.

F1 = L2 / (L1 + L2) * FT
F2 = L1 / (L1 + L2) * FT


When L1 = L2 (when the clamping stud is exactly halfway between the clamping points), F1 = F2 = FT

SAE stud sizes
Dry Threads K = 0.20
Stud size Torque (ft-lbs) Applied load (lbs)
1/4" - 20 4 1190
6 1790
8 2380
10 2980
5/16" - 18 14 3250
16 3750
18 4180
20 4640
3/8" - 16 24 4580
28 5350
32 6110
36 6880
1/2" - 13 60 8470
65 9180
70 9880
75 10590
5/8" - 11 125 13980
135 15100
145 16220
155 17340
3/4" - 10 200 18390
225 20690
250 22990
275 25280
7/8" - 9 350 27390
375 29340
400 31300
425 33260
1" - 8 450 30740
550 37580
650 44410
750 51240
Lubricated Threads K = 0.15
Stud size Torque (ft-lbs) Applied load (lbs)
1/4" - 20 4 1590
6 2380
8 3180
10 3970
5/16" - 18 14 4330
16 4950
18 5570
20 6190
3/8" - 16 24 6110
28 7130
32 8150
36 9170
1/2" - 13 60 11290
65 12230
70 13170
75 14120
5/8" - 11 125 18640
135 20130
145 21620
155 23120
3/4" - 10 200 24520
225 27580
250 30650
275 33710
7/8" - 9 350 36520
375 39130
400 41730
425 44340
1" - 8 450 40990
550 50100
650 59210
750 58320

Note: Values in the charts above are based on theoretical values. The chart values are meant to be guidelines in determining equivalent hydraulic cylinders for an application, but are by no means exact. Factors such as lubrication, material, plating and method or torque application can affect the actual clamping force. Please use proper engineering practices when designing a fixture.
  

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