A good friend of mine just asked the following question…
Is there an industry standard for how much torque a set up person will use to clamp a mold? I know there are specs for the bolt used but I think they’re too high. We use multiple clamps and multiple bolts per mold so the load is distributed all around. Our smallest machine is 55 tons and our biggest one is 500 tons. If you know of any standard, please let me know. Thanks in advance.
For your molds, we typically recommend around 50-60 ft-lbs… If the diameter of the platen bolts is larger on your biggest machines, you may be able to go a little higher. The two main factors would be the weight of the mold, and the condition of the platen… many industry people would recommend generic values around 80-120 but they usually assume ideal situations.
Keep in mind, the clamp functions by stretching a bolt to apply a bending force to the clamp which is held back by the platen threads. As a result, the more torque you use, the quicker one of these will fail (this failure is almost always found in the platen threads). This why we recommend a conservative torque value with additional clamps used when necessary.
It is always a good idea to have a specific torque value to be used at your facility. Much time can be lost when a platen bolt hole becomes stripped and needs repair.
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Can we use a pneumatic wrench to tighten the bolts?
Although many pneumatic torque wrenches are available, they will not be as consistent as a real torque wrench. Many of these wrenches can be adjusted very easily. If this occurs, even by mistake, it will either make the bolts too loose, or rip the threads right out of the platen. Manual torque wrenches are much more reliable… plus the person tightening the bolt can feel right away if the setting is too high or low. -Andy
To understand bolt torque, one needs to understand what torque is and how it is determined. Torque is the stretching of the bolt and platen threads to maximize the strength of the fastener, or mold clamp. The bolt torque will be much higher then the maximum torque requirements of the platen, so the machine torque specifications are important to follow. Often, mold setup staff will have a false sense of security and will over tighten the clamp bolts believing the clamps will hold more weight. This is actually opposite of what is occurring. The strength of the mold clamp declines when the bolt is torqued over the torque specification. The over torquing of the clamp bolts strip the platen threads and actually raises the area around the threads and may cause the mold to sit unparallel in the platen. Remember, once you have over torqued bolt threads, the threads do not relax to original condition. This is what causes gaulding of the threads.
Thank you for your comments. Very few people really understand or consider that the torque of a bolt is an artifact of deformation.They also tend to forget that the platens are cast… providing very reduced strength. -Andy
Ahem…, this post has highlighted lot of misconceptions about bolts and bolting in this industry. Perhaps I can shed some light:
As bolts are tightened, they stetch. A bolt’s natural inclination is to return to its original length (if it hasn’t been yielded). It’s precisely this “spring characteristic” which provides clamp load (bolt clamp, not mold clamp) to keep the mold affixed to the platen. As long as one knows how much bolt stress is required and has a means of verifying this load, there should be no issues whatsoever of either over-tightening or under-tightening.
Trying to control bolt clamp load by controlling torque is folly. Not only is this why many operators experience bolt failure, it’s also why flashing and shorting is often experienced in presses where the tie rods have merely been torqued. Here’s a link explaining the dangers of this particular and pervasive misconception: http://www.heviitech.com/Hevii_TorqueDanger.html
With regards to the comments about pneumatic torque wrenches: Don’t confuse pneumatic impact guns with pneumatic torque wrenches! The latter are indeed “real torque wrenches”. Their torque output is controled by adjusting the input air pressure. Each tool has a specific air-pressure to torque characteristic. Proper calibration ensures a close correlation between air pressure and torque. Impact wrenches have no such control. NO torque wrench, either pneumatic, manual, hydraulic or electronic exhibits practical torque-to bolt load relationships.
Regarding manual torque wrenches, consider, for a moment, what would happen if the installer didn’t notice that the bolt thread was damaged. In the course of tightening the fastener, the damaged threads would deform and the fastener would stop turning within the hole. The installer would feel the “correct” resistance and think that the bolt is tight. Since we now know that a bolt is only “tight” when it has been stretched sufficiently to provide the proper clamping load, it should be easy to see that this “properly tightened” bolt would be loose! Fatigue failure due to under-tightening of the bolt would cause this platen to come loose. Here’s a link to a case history in the mining indstry which may help to explain this. It’s a much larger application but, the principles are the same: http://www.heviitech.com/Hevii_CaseHistory6_b.html
Indeed, “very few people understand bolts”. Hopefully this has provided some food for thought 🙂
Thank you HG.
I understand you are a representative from Hevii Technology, though your comments will help many of our readers.
To clarify for our readers, the bolts are significantly harder than the typical cast molding machine platen.
This helps alot the company i am working for is new Start up Plant we got in to a discussion about Torque bolts on Clamps on new Toshiba Presses the bolt is rated at 401 Ft lbs I know this to high but do not remember the formula and am having trouble finding it on the web. this is a start to help show that the bolt torque rating is not the way to go
Most people do not post the values because of potential liability. If you tell me the diameter of the bolt, I can let you know what other molders tend to use for torque values.
Thanks the bolt sizes are 16 & 20 mm
Typically, we have seen companies use torque values ranging between 50-80 ft-lbs for such bolts. Essentially all this depends on the quality of the bolt holes and weight of the mold.
I FULLY APPRECIATE WHAT YOU’VE EXPLAINED TO US ALL. I STRUGGLE WITH THE RELATIONSHIP BETWEEN THE BOLT AND THE CAST MACHINE PLATENS THOUGH. IF THE GIVEN BOLT IS HARDER THAN THE PLATEN, THEN THE FORCE NEEDED TO STRETCH THE BOLT TO MAXIMIUM STRENGTH MAY OR MAY NOT EXCEED WHAT THE PLATEN THREADS CAN HANDLE. HOW CAN WE DETERMINE THIS? IT WOULD SEEM TO ME THAT THE PLATEN THREADS WOULD BE SATISFIED MUCH SOONER THAN THE BOLT ITSELF, AND THE BOLT WILL BE SATISFIED WELL AFTER THE PLATEN THREADS HAVE SEEN TOO MUCH STRESS….aNDY ANY INSIGHT?
BTW I’M NEW TO THIS BLOG AND REALLY APPRECIATE ANY INFORMATION ON THIS AND FUTURE POSTS OF MINE. OUR INDUSTRY NEEDS PLACES LIKE THIS TO GO TO!
You are correct about the platen wearing out first.
There are calculators for this type of calculation such as:
Thank you Andy for that.
Now if I can just get these values to plug in…The manufacturer we get our bolts from do not advertise the bolt grade….I’ll have to do some digging around.
Since the bolt is much stronger, the accuracy of the values for the bolt will be less important than the platen.
This is really valuable discussion. Do you know any wrentch brand suitable for this job? We are searching but not yet found good one.
Although some pneumatic torque wrenches claim to have high accuracy, the most common wrench is a manual torque wrench. These are very common in the automotive industry and should be found at a mechanics tool supplier if not at your own industrial supplier.