What is clamping pressure?

     
     
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 Clamping pressure on a workpiece 

The holding power of a clamp is an important factor to consider before beginning any application.

 

The right amount of pressure should be placed upon the workpiece during clamping; too much may damage the workpiece, whereas too little means it could come lose. 

 
     
   

Measuring pressure 

 
 Flat surface of the jaws apply pressure 

The jaws of a clamp have flat surfaces and these are the parts that will apply the pressure.

 

Having the right amount of clamping pressure is especially important for applications such as gluing, where a clamp is being used to hold two objects perfectly still while the glue sets. 

 
     
 Glue may run out if too much pressure is applied During gluing a clamp should merely be used to firmly, but gently, hold parts together. Excessive force is not needed; too much pressure will result in an uneven glue line, meaning the glue could run out or may not hold the parts together in the long term.  
     
 Amount of pressure that should be used for gluing 

In terms of gluing, the general guidelines on the amount of pressure needed during clamping are measured in pounds per square inch and are as follows:

 

  • 100 to 150 PSI for softwoods 

  • 175–250 PSI for hardwoods

 
     
 The amount of pressure also depends on the shape of the workpieces The amount of pressure needed also depends on the shape of the workpieces. If they fit parallel together then less pressure will need to be applied. On the other hand, if two pieces are slightly disjointed then the user may determine that more pressure needs to be applied. 
     
   

Pressure and force 

 
 Pressure is the force per unit area 

Pressure and force are not the same thing.

 

Pressure is defined as the exertion of force by one object on the surface area of another.

 
     
 Clamp with small jaws can cause more pressure 

To put it simply, if a clamp has small jaws, more force will automatically be applied once they are closed. This will result in a large amount of pressure being channelled on to a small surface area of the workpiece, which could cause damage.

 
     
 Jaw pads should be used to distribute the pressure 

To prevent marring the surface of the workpiece, jaw pads with a wider surface area can be used to distribute the force over a greater area, and decrease the amount of pressure that is applied to workpiece. 

 
     
   

How much pressure is a clamp capable of?

 
 Screw clamps have the largest clamping force 

Generally, screw clamps will have a greater clamping force than quick-release clamps, due to the screw mechanism.

 
     
 The clamping force of a clamp is measured in newtons 

The majority of clamps will state their maximum clamping force on the packaging. This is measured in newtons.

 

Using this information, along with the size of the clamp, a user can work out the degree of pressure the clamp will exert on a workpiece. There is no specific rule for the amount of pressure that is needed for each clamping application or workpiece.

 
     
 Multiple clamps is like using many hands 

However, as a guide a user should consider the larger the workpiece, the greater the clamping pressure needed. If a clamp does not seem large or strong enough to apply the amount of pressure needed, multiple clamps can be used to increase this pressure.

 

Just think of it as 'many hands make light work!'. 

 
     
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