20 or 25 mm for the Ballscrew Diameter?

[pauloms]

Hi Guys,

I tried search the forum to decide this, but each case is a new case and I need some help.

I'm building my cnc project. The dimensions will be 1200x800x360 mm. I'm thinking to use 25 mm ballscrews with a lead of 10 mm and belt reductions of 2-1. Additionally, I intend to use 23 kgf step motors.

I'm undecided with the ballscrews diameter. 25 mm will be weighty to these motors and 20 mm will be just fine or not a problem?

BEst Regards
Paulo

[Zappautomation]

Why do you need such large diameter ball screws?
Also what motors are you thinking of using? you say 25kgf motors, but kgf is not a measure of torque, do you mean kg-cm?
If you mean 25Kg-cm, then this is 2.4Nm and with this you will not need anything larger than 16mm diamter?
Is it the speed and whipping that you are worried about?
A 16mm ball screw that is 1200 long has a critical speed of about 1100 RPM.

[pauloms]

Originally Posted by Zappautomation Why do you need such large diameter ball screws? Also what motors are you thinking of using? you say 25kgf motors, but kgf is not a measure of torque, do you mean kg-cm? If you mean 25Kg-cm, then this is 2.4Nm and with this you will not need anything larger than 16mm diamter? Is it the speed and whipping that you are worried about? A 16mm ball screw that is 1200 long has a critical speed of about 1100 RPM.

Thanks for your answer!

You are correct, I wanted to say 23 kg.cm. with the reduction 2-1 it will have 46 kg.cm. BEcause of this reduction I believe that a lead of 10 mm is important.

My machine must be robust and prepared to work with aluminum. I must say, my idea of using ballscrews with a 25 mm of diameter came from looking other projects with identical dimensions of free working area. The other reason is because a higher charge capacity ballscrew will last more.

But I don't have any experience and sufficient knowledge...


REgards
Paulo

[Zappautomation]

There is no point adding a 2:1 ratio then doubling the lead of the screw, this will infact give you less thrust than going direct and having a 5mm lead on the screw.
If we work on 2.3Nm of torque at the motor with a direct drive to the ball screw and a 5mm lead, the thrust is 59Kg.
This is way within the capabilities of the 16mm ball screw.
have a look at the attached file.
Also, if the stepper motor is rated as 23Kg-cm, this is only the holding torque and the actual torque while running will be a lot lower and will depend on the driver you are using and voltage.

Originally Posted by pauloms Thanks for your answer! You are correct, I wanted to say 23 kg.cm. with the reduction 2-1 it will have 46 kg.cm. BEcause of this reduction I believe that a lead of 10 mm is important. My machine must be robust and prepared to work with aluminum. I must say, my idea of using ballscrews with a 25 mm of diameter came from looking other projects with identical dimensions of free working area. The other reason is because a higher charge capacity ballscrew will last more. But I don't have any experience and sufficient knowledge... REgards Paulo

[pauloms]

Originally Posted by Zappautomation There is no point adding a 2:1 ratio then doubling the lead of the screw, this will infact give you less thrust than going direct and having a 5mm lead on the screw. If we work on 2.3Nm of torque at the motor with a direct drive to the ball screw and a 5mm lead, the thrust is 59Kg. This is way within the capabilities of the 16mm ball screw. have a look at the attached file. Also, if the stepper motor is rated as 23Kg-cm, this is only the holding torque and the actual torque while running will be a lot lower and will depend on the driver you are using and voltage.

I intend to use, for now, the cnc3ax. Then, I will see. The same for the motors. First, I want to see the machine working...


What are the differences between SFU and SFI Ballscrews?

REgards
Paulo

[Zappautomation]

Only the mounting on the ball nut.
The FSI has four holes and the FSU has six.

Originally Posted by pauloms I intend to use, for now, the cnc3ax. Then, I will see. The same for the motors. First, I want to see the machine working... What are the differences between SFU and SFI Ballscrews? REgards Paulo

[pauloms]

Originally Posted by Zappautomation Only the mounting on the ball nut. The FSI has four holes and the FSU has six.

Thanks.

I saw your website. I intend to use BK and BF supports. My concern is about the maintenance that the bearings of these supports need. Is this something to consider in the mechanical project to facilitate the access to the end supports? Or is the periodical maintenance (with a regular work) of several months?

Regards
Paulo

[Zappautomation]

The BK comes with a seal on front and back so this will stop dirt getting into the bearing.
Generally they don’t need a lot of servicing, but if you application is particually demanding the BK is also available with a grease nipple so you can service the bearings quite easily.

Originally Posted by pauloms Thanks. I saw your website. I intend to use BK and BF supports. My concern is about the maintenance that the bearings of these supports need. Is this something to consider in the mechanical project to facilitate the access to the end supports? Or is the periodical maintenance (with a regular work) of several months? Regards Paulo

[pauloms]

Originally Posted by Zappautomation The BK comes with a seal on front and back so this will stop dirt getting into the bearing. Generally they don’t need a lot of servicing, but if you application is particually demanding the BK is also available with a grease nipple so you can service the bearings quite easily.

If I choose the normal version, will it be necessary to dismount the seals on both sides to grease the bearings?

Regards
Paulo

[Zappautomation]

For proper servicing grease needs to be injected between the two bearings so it goes into both bearings.
The Q option (Grease nipple) does not cost that much more, so if ease of servicing is something you need, then this is the option for you.

Dismantling the support unit every few months is not be an option i would suggest.

[H.O]

There is no point adding a 2:1 ratio then doubling the lead of the screw, this will infact give you less thrust than going direct and having a 5mm lead on the screw. If we work on 2.3Nm of torque at the motor with a direct drive to the ball screw and a 5mm lead, the thrust is 59Kg.

Agreed on the reduction vs screw lead - no use in going 2:1 and doubling the lead.... However the thrust calculation seems a bit off. My calcs gives 2889N or 295kg of thrust for 2.3Nm on a 5mm lead (100% effective). (2.3 / 0.005/2PI)

/Henrik.

[pauloms]

Originally Posted by Zappautomation For proper servicing grease needs to be injected between the two bearings so it goes into both bearings. The Q option (Grease nipple) does not cost that much more, so if ease of servicing is something you need, then this is the option for you. Dismantling the support unit every few months is not be an option i would suggest.

Yes, I believe that the GRease nipple is important.

But, is it not necessary to adjust the nuts (7 and 6 in the draw of BK supports) times to times?

Regards
Paulo